Move custom shaders guide into the Documentation directory

[ptrgags]

Custom Shaders does have documentation, it just was hiding in the ModelExperimental folder while under development. I'm moving it to a more discoverable place, in Documentation/Contributors/CustomShaderGuide/README.md.

@ggetz I'm not 100% certain if this is the right subfolder. If there's a better place to put it, please let me know and I'll update this PR.

[cesium-concierge]

Thanks for the pull request @ptrgags!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[ggetz]

I agree that its a bit out of place to put this in Documentation/Contributors. To be consistent with the only other non-contributor documentation, the fabric schema, I think this should live in Documentation directly, so maybe Documentation/CustomShaderGuide/README.md?

[ptrgags]

@ggetz updated! I also added a changelog entry

[ggetz]

Thanks @ptrgags! So this guide's content has been previously reviewed, correct?

[ptrgags]

@ggetz yes, this guide was reviewed when Custom Shaders was originally added, and again recently in #10018