docs: Simplify the top level of documentation

[claremacrae]

Types of changes

Changes visible to users:

• Documentation (prefix: docs - improvements to any documentation content for users)

Description

Some small rearrangements to the user docs to:

• Reduce the number of pages in the top-level
• Simplify the Introduction page.

All but the last few changes were made jointly in a pairing session with @aubreyz, who spotted the potential for improvements.

Motivation and Context

See:

• Documentation ordering #1693

How has this been tested?

• By reviewing the edits in Obsidian
• By publishing the pages to check the effect.

Screenshots (if appropriate)

Checklist

• My code follows the code style of this project and passes yarn run lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

Terms

• My contribution follows this project's contributing guide
• I agree to follow this project's Code of Conduct

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[claremacrae]

Note that at some point I stopped adding links to the following page, as it was slowing us down:

contributing/Documentation/Redirects - Testing old links still work.md

However, old links to all the moved and removed pages will still work, due to the addition of aliases.