Add helpers for authenticated media, and associated documentation

[turt2live]

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).

[turt2live]

for the reviewer - I think this our first usage of the blockquote styles provided by github. See https://github.com/orgs/community/discussions/16925 for details.

[dbkr]

How will we manage the second transition from unstable to stable?

[turt2live]

Ah, sorry, it's mentioned briefly in the README diff here and in the depths of element-hq/element-web#27326 (comment) somewhere - we'd be introducing a breaking change later (once the MSC is either stable or released) which only returns authenticated media URLs.

I'm not sure there's much of a way to avoid a breaking change here, given we'd want the js-sdk to produce authentication-required URLs by default as soon as it possibly can.

[dbkr]

Fair enough, as long as there's a plan for managing the second transition (ie. to stable) as well.