Add a link id for checking out docs

[tiffany-taylor]

To make it easier to externally link to the instructions for checking out the docs. Probably could be argued that each header should have a link id... I can supply that change if requested.

[jimwins]

New home for docs has a similar anchor. Will be https://doc.php.net/guide/local-setup.md#without-docker, can be previewed for now at https://doc-new.php.net/guide/local-setup.md#without-docker