Remove duplicated CONTRIBUTING information

[alice-i-cecile]

This information is now found at https://bevyengine.org/learn/contribute/introduction/. We should remove:

1. https://github.com/bevyengine/bevy/blob/main/CONTRIBUTING.md
2. https://github.com/bevyengine/bevy/blob/main/docs/release_checklist.md
3. https://github.com/bevyengine/bevy/blob/main/docs/the_bevy_organization.md
4. https://github.com/bevyengine/bevy/tree/main/.github/contributing

While we're there, we should also be sure that the README links to the Contributing Guide.

[TrialDragon]

Should they be out right removed, or just have a "This is now located here (page link)." text replace them more or less? Or should that only happen with CONTRIBUTING.md, or specific ones? Mainly thinking so that old links don't break on people.

[alice-i-cecile]

Only with CONTRIBUTING.md IMO. Everything else should just be removed.

[TrialDragon]

Note

We should also modify github-actions bot's "new contributor" message to link to https://wwww.bevyengine.org/learn/contribute/introduction rather than CONTRIBUTING.md.