document POST /bookmark/{gameId}

[jadddon]

closes #633 (Add docs for toggling bookmarking your own games #633)

documents POST /bookmark/{gameId}, which was opened to the preference:write OAuth scope in lichess-org/lila@5820514.

the following is taken from the lila source:

• toggles the bookmark on a game for the authenticated user, returning 204 No Content.
• optional v query boolean sets the bookmark explicitly instead of toggling it, making the request idempotent.
• requires an OAuth token with the preference:write scope.

description cross-links to the existing export your bookmarked games endpoint.

testing:
build, lint, spectral, scalar-lint and check-format all pass locally.