Update the Docs landing page for the CodeQL docs site

[felicitymay]

These updates reflect the migration of documentation for CodeQL CLI and CodeQL for VS Code to the GitHub docs site.

In addition, the following changes are included:

• Heading levels on the landing page adjusted for accessibility (no skipped levels)
• Minor changes to the footer on the landing page to avoid redirects in links to content that's moved
• New article on Running CodeQL queries written and added to the landing page
• Template updated to match changes to the landing page

[felicitymay]

To aid with reviewing, here are a couple of screenshots:

New docs landing page and revised drop-down menu:

Revised footer for the docs landing page:

New article:

[coadaflorin]

This looks good to me. I was mostly reviewing the images you shared (Thank you for doing that!).

[jc-clark]

This looks great from a docs content perspective! Thank you for the screenshots, those were very helpful ❤️

[felicitymay]

@coadaflorin or @jc-clark - can I ask for another approval? Committing Joe's review suggestions dismissed your reviews.

I think that the update to the docs landing page will be published when we release CodeQL CLI 2.19.1.