Add docs for todo() method

[bertheyman]

The announcement of Pest 2 mentions the ->todo() method, but links to a section of the docs that doesn't exist.

Other pages in the docs seem to use #content-topic for links, connected to a ## topic.
As such, I tried to match the link in the Pest 2 announcement, linking to #content-creating-todos:
https://pestphp.com/docs/announcing-pest2

[bertheyman]

Check indicate a timeout on a license link that not changed in this pull request.
Might have been a temporary issue, I get a normal result when I check it now:
https://www.gnu.org/licenses/agpl-3.0.en.html