Move the js sdk documentation to the project

https://matrix.org/docs/legacy/usage-of-the-matrix-js-sdk/ should at least route to https://github.com/matrix-org/matrix-js-sdk#quickstart, and maybe should be ported entirely to a github hosted mdbook?

Start with the legacy usage-of-the-matrix-js-sdk URL and compare it with the matrix-js-sdk GitHub Quickstart. The issue leaves open whether to add a redirect or port the documentation to a GitHub-hosted mdBook, so confirm the intended scope before changing anything. Done means the legacy page routes correctly or the agreed documentation is hosted in the project.

Written by the indexing model from the issue text.