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

[bertheyman]

@owenvoke Anything I can adapt here to make this a good addition to the docs?
Link checker failed, I remember it complained about a line in a file that wasn't touched in this PR - could be something that was temporary down? Not sure if I can manually trigger the check again.

[owenvoke]

LGTM.

[owenvoke]

Can you re-push the branch to re-run the CI? If not, I'll re-run it tomorrow morning.

[bertheyman]

Sure thing! I've updated the branch