[Docs] Extract 'Layer Change Code' and EEPROM

[coliss86]

Description

As described in https://github.com/qmk/qmk_firmware/issues/18089, here is a proposition of a reorganization of the custom_quantum_functions.md page. I have also added a "legacy" content section as in feature_advanced_keycodes.md

Types of Changes

• [ ] Core
• [ ] Bugfix
• [ ] New feature
• [ ] Enhancement/optimization
• [ ] Keyboard (addition or update)
• [ ] Keymap/layout/userspace (addition or update)
• [x] Documentation

Issues Fixed or Closed by This PR

• https://github.com/qmk/qmk_firmware/issues/18089

Checklist

• [ ] My code follows the code style of this project: C, Python
• [ ] I have read the PR Checklist document and have made the appropriate changes.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[spidey3]

I think this is a good start, but I wonder, should all of the subsections should be broken out, and this page just turned into an index...

[coliss86]

I think this is a good start, but I wonder, should all of the subsections should be broken out, and this page just turned into an index...

I'm sorry if I misunderstood:

• do you ask to split the page into several pages ? or
• you think that this page will become an index and it is not a good idea ?

[spidey3]

• do you ask to split the page into several pages ? or
• you think that this page will become an index and it is not a good idea ?

Sorry - I didn't mean that to be a call to action, but more of a statement of fact, that eventually this page could just become an index (but not in this PR).