New "Glossary" page

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#reporting-new-issues).

### Description

We should have a dedicated page for all "jargons" used in the Docusaurus site, especially those UI-related. For example:

- Category link
- Category generated index page
- Docs sidebar
- Navbar
- Table of contents
- Footer
- Mobile sidebar (which is just the navbar)
- Mobile secondary menu (the one populated by the doc sidebar)
- Color mode toggle
- Much, much more

As well as some abstract technical terms like:

- Swizzling
- Plugins/themes/presets
- Theme config
- Bundler, transpiler, minifier

The UI-related terms would need some screenshots about which specific regions we are referring to. The aim is to create a unified lexicon in the community and make people's questions less ambiguous. Another gain is that some of these terms are quite hard to translate (for example, "preset" doesn't have a natural Chinese translation), so having a dedicated keyword-definition page allows people to be exposed to these unnatural terms.

This should be unversioned, so it could probably be a standalone Markdown page.

### Self-service

- [X] I'd be willing to address this documentation request myself.