Add RTD pull request preview

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This PR demonstrates a working build on RTD, with preview output.

For automatic pull request previews you also need to toggle a setting in RTD:

To get this working on the main repo

1. Open a PR with the contents of .readthedocs.yaml in the main repo (now you can't move PR target?)
2. Sign up for a Community RTD account
3. Connect the RTD account with GitHub
4. Add the Guidelines repo to RTD, it'll pick up the config file
5. Toggle the PR build setting

And I'll delete the temporary Guidelines RTD account I used.

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It's possible we should build these with --ignore-spec-lock-diff by default.

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This happened!