Deprecate old contributing documentation / information (#14885)

# Objective
Fixes #14884

We have launched the new [contributing
guide](https://bevyengine.org/learn/contribute/introduction) 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.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

.github/workflows/welcome.yml

@@ -41,5 +41,5 @@ jobs:
  repo: context.repo.repo,
  body: `**Welcome**, new contributor!
 
- Please make sure you've read our [contributing guide](https://github.com/bevyengine/bevy/blob/main/CONTRIBUTING.md) and we look forward to reviewing your pull request shortly ✨`
+ Please make sure you've read our [contributing guide](https://bevyengine.org/learn/contribute/introduction) and we look forward to reviewing your pull request shortly ✨`
  })