chore: improve sphinx workflow with composite action

[Venefilyn]

This changes the Sphinx GitHub action to use a composite action that does not install Sphinx on its own, but rather takes care of comments so that we can more easily run everything as we want

We also add packages using apt instead of pip, as we otherwise ran into some weird issues. But I've tested on my branch and works fine

See https://venefilyn.github.io/convert2rhel

Jira Issues:

-

Checklist

• [ ] PR has been tested manually in a VM (either author or reviewer)
• [ ] Jira issue has been made public if possible
• [ ] [RHELC-] or [HMS-] is part of the PR title
• [ ] GitHub label has been added to help with Release notes
• [ ] PR title explains the change from the user's point of view
• [ ] Code and tests are documented properly
• [ ] The commits are squashed to as few commits as possible (without losing data)
• [ ] When merged: Jira issue has been updated to Release Pending if relevant

[has-bot]

This PR does not require integration tests to be run.

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[codecov[bot]]

Codecov Report

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

Project coverage is 95.83%. Comparing base (3c8e4a3) to head (81f2d89).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1274 +/- ## ======================================= Coverage 95.83% 95.83% ======================================= Files 55 55 Lines 4756 4756 Branches 836 836 ======================================= Hits 4558 4558 Misses 112 112 Partials 86 86 ``` | [Flag](https://app.codecov.io/gh/oamg/convert2rhel/pull/1274/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oamg) | Coverage Δ | | |---|---|---| | [centos-linux-7](https://app.codecov.io/gh/oamg/convert2rhel/pull/1274/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oamg) | `91.02% <ø> (ø)` | | | [centos-linux-8](https://app.codecov.io/gh/oamg/convert2rhel/pull/1274/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oamg) | `91.95% <ø> (ø)` | | | [centos-linux-9](https://app.codecov.io/gh/oamg/convert2rhel/pull/1274/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=oamg) | `92.00% <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=oamg#carryforward-flags-in-the-pull-request-comment) to find out more.

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