docs: avoid hiding content in section index pages, de-dupe project spec overview

[awphi]

Background

One of my pet peeves with docusaurus is its default behaviour to render the content of index.md or README.md in subdirectories as a page tied to the section heading of that subdir.

For example, currently our introduction/README.md content is rendered under this clickable 'Overview' section heading:

Imo, this harms discoverability of important overview content. Now take a look at (again, imo) a couple of good docs sites with docusaurus-like sidebars:

• https://vite.dev/guide/
• https://docs.astro.build/en/concepts/why-astro/
• https://rollupjs.org/browser/
• https://mise.jdx.dev/installing-mise.html

None of them do this! I prefer the cleanliness of the site feeling like a directory of files and avoiding the directories holding content of their own.

Changes

• Sets some docusaurus metadata in _category_.json files to avoid the gripe outlined above.
• Deletes the 00-overview.md page of the project specification. It felt there was not useful content on there not covered in the README and having two overview/introduction type pages was a bit odd. Lmk if you disagree!

Old sidebar (section headings are clickable and contain README.md content):

New sidebar (section headings are not clickable!):

Checklist

• 🤖 This change is covered by tests as required.
• 🤹 All required manual testing has been performed.
• 📖 All documentation updates are complete.