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.

Built HTML and checked output.