Docs: Add `abilities_api_init` to list of actions

[jonathanbossenger]

Raised in https://github.com/WordPress/abilities-api/pull/102/files#r2416117160.

We are missing abilities_api_init covered in the Hooks document. That one is fundamental to explain the ability registration. It's indirectly covered elsewhere, but it would be still nice to have it explicitly listed.

[jonathanbossenger]

#117 includes a fix for this