This change updates the extension static media documentation to explain what is needed to enable the use of TypeScript and ES6 modules (as well as ES6 more generally) in extensions. The only part that's still missing is an explanation of how to import code from the Review Board/Djblets JavaScript codebase. I'm undecided if that's something we should document now (and perhaps enable via some symlinks like `.reviewboard`/`.djblets` during the `setup.py develop` process), or if we should wait until more of those codebases have been converted to modules. Testing Done: Built HTML and checked output.