Remove underscores from docs page names

[stichbury]

Description

This PR changes names as #313 describes.

Screenshot

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[antonymilne]

@stichbury in case you didn't see it, we have RTD builds done on every PR so you can check rendering easily.

Also would it make sense if we add you to CODEOWNERS for docs? So you automatically get tagged as reviewer for relevant PRs.

[stichbury]

Oh, whoops, sorry! I didn't mean this to be ready for review yet, it should have gone into draft. Apologies, but thanks for the feedback as that'll make it easier for me to find and fix those issues in the morning. I didn't change any formatting so am not sure how that's happened but will roll it back so it's just file renames in this PR 🤦

Yes to CODEOWNERS too please!