Deprecate old contributing documentation / information

[TrialDragon]

Objective

Fixes #14884

We have launched the new contributing guide on Bevy's website, so these sources of information should be removed to avoid syncing across duplicate files and maintaining a single source of truth on contributing.

Solution

Remove the files:

• docs/release_checklist.md.
• docs/the_bevy_organization.md.
• .github/contributing/engine_style_guide.md.
• .github/contributing/example_style_guide.md.

These are replaced by:

• https://bevyengine.org/learn/contribute/project-information/release-process/.
• https://bevyengine.org/learn/contribute/project-information/bevy-organization/.
• https://bevyengine.org/learn/contribute/helping-out/opening-pull-requests/#style-guide.
• https://bevyengine.org/learn/contribute/helping-out/creating-examples/#style-guide

Make CONTRIBUTING.md re-direct to Bevy's website's Contributing Guide https://bevyengine.org/learn/contribute/introduction

Change the contributing guide link in welcome.yml workflow to link to Bevy's website's Contributing Guide https://bevyengine.org/learn/contribute/introduction

Testing

I looked at the markdown files in my repository's branch to make sure they look fine. I have not tested the welcome.yml workflow since I don't know how, without having a new contributor make a PR to my branch.

[TrialDragon]

Blocked on bevyengine/bevy-website#1614 (straight-forward), bevyengine/bevy-website#1615 (straight-forward), and bevyengine/bevy-website#1616 (trivial) so that the content is there properly before it is removed here.

Edit: Unblocked 🎉