Add test to check for uppercase letters in dir and file names

What does this MR do and why?

Add a test in lint-doc.sh to check for uppercase letters in directory and file names under the doc/ directory. According to https://docs.gitlab.com/ee/development/documentation/site_architecture/folder_structure.html#work-with-directories-and-files:

When working with directories and files:

• Do not use special characters and spaces, or capital letters in file names, directory names, branch names, and anything that generates a path.

How to set up and validate locally

1. Check out this branch.

2. Create a few test directories:

   mkdir doc/Test{,A,B,C}

3. Run the script:

   scripts/lint-doc.sh

   The error should be like the following:

   => Checking for directory names containing an uppercase letter...
   
   ✖ ERROR: Found one or more directories with an uppercase letter in their name. Use lowercase instead of uppercase for the directory names.
   https://docs.gitlab.com/ee/development/documentation/site_architecture/folder_structure.html#work-with-directories-and-files
   
   doc/TestB
   doc/TestA
   doc/Test
   doc/TestC
   
   => Checking for file names containing an uppercase letter...
   
   ✖ ERROR: Found one or more file names with an uppercase letter in their name. Use lowercase instead of uppercase for the file names.
   https://docs.gitlab.com/ee/development/documentation/site_architecture/folder_structure.html#work-with-directories-and-files
   
   doc/development/sec/CycloneDX_property_taxonomy.md

   The doc/development/sec/CycloneDX_property_taxonomy.md file is a legit file that should produce an error and fail the job. It was recently added in !118823 (merged), which is why I created this MR 🙂 And here's the failed job to demonstrate that the test works in CI as well https://gitlab.com/gitlab-org/gitlab/-/jobs/4284886495. This is fixed in the second commit !120726 (5d41d891).

4. To remove the created test directories:

   git clean -df

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.