🏗📖 Auto-format all markdown files in ampproject/amphtml with Prettier

[rsimha]

The AMP Project already uses Prettier to auto-format JS files and several non-JS files across the repo. This PR adds documentation files to the mix.

Highlights:

• Includes .md files in the list of files that are checked by Prettier
• Auto-formats all .md files in the repo by running gulp prettier --fix
• Skips the broken links check for PRs that modify a large number of .md files
• Prints suggested fixes when formatting errors are detected
• Exempts GitHub issue templates (they use a custom header that's incompatible with Prettier)
• Make extension generator (gulp make-extension) resilient to markdown file auto-fixing

Notes:

• Most of these changes do not affect the rendered HTML. Exceptions are:
  • Attribute lists are changed to all-in-one-line if they all fit on one line
    
  • Attribute lists are changed to one-per-line if they don't all fit on one line
    
• Once this PR is merged, .md files will have to satisfy Prettier's formatting requirements
• A workflow to make your IDE auto-fix .md files on save is available here
• An alternative is to manually run gulp prettify --local_changes --fix

Example of a failing check:

Fixes #25148

[rsimha]

The link checker is reporting several seemingly broken links in our documentation ☹️ On taking a closer look, most of these failures are due to requests to amp.dev timing out, probably because of the sheer number of requests being made by the checker.

I don't think we should be running the broken links check for PRs that modify a large number (> 20) of markdown files, so I've skipped it for large PRs. The check will continue to run when a smaller number of files are touched.

[rsimha]

I sanity checked several of the changes in this PR, and found no visible changes to the rendered HTML in most cases. This is now ready for review.

[rsimha]

Rebased and added a function to print suggested fixes for badly formatted .md files.

[mrjoro]

overall changes look fine (with a few comments) though I admit to not reviewing all 483 files in detail :)

[rsimha]

Rebased, reverted changes to issue template headers, and exempted them from future checks.

[rsimha]

Some final notes:

• Self-reviewed every file in contributing/, and most files in ads/, extensions/, etc.
• Found and fixed a handful of existing bugs that were exposed by auto-formatting (tabs instead of spaces, bad formatting, etc.)
• Added prettier-ignore overrides where required
• Verified that the majority of .md files have no visible changes when rendered
• Tested VSCode auto-fix-on-save workflow for .md files
• Tested gulp prettify with --fix, --files, and --local_changes

Merging this now 🤞