Fixes Javadocs build & adds workflow to ensure no more breakages

[WalshyDev]

Description

The javadocs have again been broken for a while, I said I wanted to do this last time and forgot. So now, we're gonna both fix the docs and add validation to this repo.

Proposed changes

• Adds a new workflow "Javadocs" to validate that we can build the javadocs and there are no errors
• Fixes the javadoc errors in ThreadService
• Changes the destination dir of the javadocs since we now have a real "docs" dir
  • Accompanying javadocs PR - Update dir based on Slimefun/Slimefun4#4168 javadocs#14

Related Issues (if applicable)

N/A

Checklist

• I have fully tested the proposed changes and promise that they will not break everything into chaos.
• I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• I have made sure that the proposed changes do not break compatibility across the supported Minecraft versions (1.16.* - 1.20.*).
• I followed the existing code standards and didn't mess up the formatting.
• I did my best to add documentation to any public classes or methods I added.
• I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• I added sufficient Unit Tests to cover my code.

[github-actions]

Your Pull Request was automatically labelled as: "✨ Fix"
Thank you for contributing to this project! ❤️

[sonarcloud]

Quality Gate passed

Issues
8 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions]

Slimefun preview build

A Slimefun preview build is available for testing!
Commit: 85883036

https://preview-builds.walshy.dev/download/Slimefun/4168/85883036

Note: This is not a supported build and is only here for the purposes of testing.
Do not run this on a live server and do not report bugs anywhere but this PR!

[JustAHuman-xD]

Workflows are in my realm of little knowledge, from looking at it it looks good but im also not a professional in this area 😅