Update extension based SP with content from main (#289)

[SecondSkoll]

• docs: Update customise.rst
• chore: Sync all changes from main to extension branch
• fix: fix build and doc check adds to gitignore
• fix: workflows updates init script to provide dependency check
• fix: removed file
• fix: change to workflow sed
• fix: change branch to support testing when staging PRs

[SecondSkoll]

Had to do some funky stuff to get this to work, as the tests require pulling from a branch in this project. So I essentially force merged into the extension-main-combined branch, so the init script clones from there, then submitted this PR from that branch.

When this is approved, the only change will be to line 53 in init.sh to change the branch back to the use-canonical-sphinx-extension branch. The tests will then fail - because it will be pulling the wrong content (but it will be the right content once it is merged).

This is the best we can do for the moment without a reimplementation of the init script.

[akcano]

@SecondSkoll on it, will do today or tomorrow.

[akcano]

LGTM with a few suggestions.

A couple of considerations:

• We'll probably need to prune the Sphinx-specific and general dependencies and keep track of them down the road, if only because their number is growing, and I have a feeling we could do away with some.
• A link to the documentation style guide should probably accompany markup-specific guides.

[SecondSkoll]

Some suggested adjustments in response to the review - so I can batch them.