Remove features-by-stage tables

What does this MR do?

This MR removes the:

• Many features-by-stage tables on the GitLab Docs landing page.
• DevOps lifecycle graphic.

I don't think those tables are well maintained, and the "about site" (https://about.gitlab.com) does a really great job of discussing the different DevOps stages, and it even links back heavily into this docs site.

From memory, the tables were established in the first place to:

• Replace long bullet lists of features.
• Provide a way of navigating into docs, before we had the global nav.

I don't think they serve any purpose now, and just present maintenance overhead. This MR doesn't replace the tables with anything, though future docs architecture ideas might refactor it further.

Our "stages of the DevOps lifecycle" graphic is out of date:

• What it is:

  

• What it should be:

  

We could probably get the SVGs used in the "about site" (rather than converting to a long PNG), but we then have two sources of truth.

I've also taken the opportunity to remove hard-coded font awesome icons (which would otherwise need porting) and inline HTML.

So a real cleanup!

Preview

• Before: https://docs.gitlab.com/ee/README.html
• After: http://docs-preview-ee-49184.178.62.207.141.nip.io/ee/README.html

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.