Move docs from docs.wire.com to generated helper page served by brig.

[fisx]

Also updated docs.wire.com accordingly.

(I categorized this as "bug" because the data on docs.wire.com was out of date and the data on the brig page was incomplete.)

Checklist

• [x] Add a new entry in an appropriate subdirectory of changelog.d
• [x] Read and follow the PR guidelines