Update lefthook glob for docs

What does this MR do?

Related to !46517 (merged)

I recently had an MR that updated a link in qa/README.md, which should not be linted by docs, but LEFTHOOK got triggered for that change. This MR updates the glob to make sure it only triggers for markdown files in doc/. Tested locally and in GitPod, and seems to work as expected:

• qa/README.md is ignored.
• doc/README.md is linted.
• doc/ci/README.md is linted.

I inserted formatting mistakes in all three files above, and it works correctly (ignoring qa/, but catching doc/ and subdirectories):

$ lefthook run pre-push
/Users/ravlen/.asdf/installs/ruby/2.7.2/lib/ruby/gems/2.7.0/gems/lefthook-0.7.2/libexec/lefthook-mac run pre-push
Lefthook v0.7.2
RUNNING HOOKS GROUP: pre-push

 rubocop (SKIP. NO FILES FOR INSPECTION)

 eslint (SKIP. NO FILES FOR INSPECTION)

 scss-lint (SKIP. NO FILES FOR INSPECTION)

 haml-lint (SKIP. NO FILES FOR INSPECTION)

  EXECUTE > markdownlint
yarn run v1.22.4
warning ../package.json: No license field
$ markdownlint --config .markdownlint.json doc/README.md doc/ci/README.md
doc/ci/README.md:10 MD022/blanks-around-headings/blanks-around-headers Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Below] [Context: "# GitLab CI/CD"]
doc/README.md:21 MD022/blanks-around-headings/blanks-around-headers Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Above] [Context: "## Overview"]
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

  EXECUTE > vale
/usr/local/bin/vale

 doc/README.md
 20:1  error  Link "[entire DevOps                                                            gitlab.RelativeLinks 
              lifecycle](https://docs.gitlab.com/ee/index.html#the-entire-devops-lifecycle)"                       
              must be a relative link with a .md extension.                                                        


 doc/ci/README.md
 12:13  error  Link "[continuous                                                                                        gitlab.RelativeLinks 
               methodologies](https://docs.gitlab.com/ee/ci/introduction/index.md#introduction-to-cicd-methodologies)"                       
               must be a relative link with a .md extension.                                                                                 

✖ 2 errors, 0 warnings and 0 suggestions in 2 files.

  EXECUTE > danger
Results:

Messages:
- [ ] This merge request adds or changes documentation files. A review from the Technical Writing team before you merge is **recommended**. Reviews can happen after you merge.

SUMMARY: (done in 2.83 seconds)
✔️  danger
🥊  markdownlint
🥊  vale

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
  • Ensure that the product tier badge is added to doc's h1.
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group labels, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add the product tier badge accordingly.
• Add GitLab's version history note(s).
• Add/update the feature flag section.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Ensure docs metadata are present and up-to-date.
  • Ensure Technical Writing and documentation are added.
  • Add the corresponding docs:: scoped label.
  • If working on UI text, add the corresponding UI Text scoped label.
  • Add twdoing when starting work on the MR.
  • Add twfinished if Technical Writing team work on the MR is complete but it remains open.

For more information about labels, see Technical Writing workflows - Labels.

For suggestions that you are confident don't need to be reviewed, change them locally and push a commit directly to save others from unneeded reviews. For example:

• Clear typos, like this is a typpo.
• Minor issues, like single quotes instead of double quotes, Oxford commas, and periods.

For more information, see our documentation on Merging a merge request.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.