Reformat the markdown files in the `docs` directory to make scraping
with DevDocs easier and more consistent. This establishes some
conventions for documentation intended for scraping, namely that:

* the page has `#`/`<h1>` title
* headings are `##`/`<h2>`
* symbols (e.g. functions) are `###`/`<h3>`, formatted similar to
  `gir_scraper` in DevDocs (e.g. `*`/`<ul>` for args list).

Along with general documentation updates, the changes here include:

* All modules have their own page, except for overrides and the
  `Modules.md` page has been stubbed out
* `Home.md` has been stubbed out and merged into `README.md`
* Some new pages have been added for testing, profiling, ESM
* Built-ins that are deprecated now have links to alternatives