Refactor `readthedocs-preview.yml` into own workflow

[tpvasconcelos]

Ran into permission issues in https://github.com/tpvasconcelos/ridgeplot/pull/262 due to the use of the pull_request event instead of pull_request_target

[github-actions[bot]]

:warning: Missing changelog entry :warning:

Please add an entry to the changelog in docs/reference/changelog.md for this PR. The entry should be in the following format:

- Description of the change ({gh-pr}`263`)

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (e880c78) to head (a824ffc). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #263 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 13 13 Lines 528 528 Branches 69 69 ========================================= Hits 528 528 ``` | [Flag](https://app.codecov.io/gh/tpvasconcelos/ridgeplot/pull/263/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tomas+Pereira+de+Vasconcelos) | Coverage Δ | | |---|---|---| | [combined-src](https://app.codecov.io/gh/tpvasconcelos/ridgeplot/pull/263/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tomas+Pereira+de+Vasconcelos) | `99.24% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tomas+Pereira+de+Vasconcelos#carryforward-flags-in-the-pull-request-comment) to find out more.

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