MATTER-6477: Update outdated references in Matter docs

[Michael Duggan (miduggan24)]

Description

With implementation of project upgrades, we have to make changes to cater to new architecture

• Remove any references to DataModelCallbacks or app manager files (removed in refactor)
• Update any notes about customization with new flow

This PR splits locations with outdated references into two: New Architecture and Legacy Architecture. New Architecture sections follow the criteria above, while Legacy Architecture sections follow their old format. Any Legacy Architecture references will be removed in the patch once all Matter apps are refactored.

Related Issue

MATTER-6477

Changes Made

• Remove references to DataModelCallbacks file
• Anywhere that reference MatterPostAttributeChangeCallback, provide context on DMPostAttributeChangeCallback and overridability
• Remove any instances of app manager references
• Update code snippets to be up to date with new AppTask structure

Checklist

• I have read the Contributor License Agreement.
• I have followed the repository's coding guidelines .
• I have checked the action workflow results and they are all passed.

Screenshots (if applicable)

n/a (only new image seen in diff, change MatterPostAttrributeChangeCallback -> DMPostAttributeChangeCallback)

Additional Notes

n/a

[Sergei Lissianoi (selissia)]

Should there be a document or section explaining CRTP and the overall model of how apps are meant to be customized now? I think there should be (in this PR or a separate one). Can take some of the content from the READMEs.

[Michael Duggan (miduggan24)]

Should there be a document or section explaining CRTP and the overall model of how apps are meant to be customized now? I think there should be (in this PR or a separate one). Can take some of the content from the READMEs.

this was discussed in this thread: https://silabsiot.slack.com/archives/C0AG6TM9SMC/p1779828060955649
once we have all apps refactored in the patch, we have a docs task open to migrate some common README info into one docs page
see here: https://jira.silabs.com/browse/MATTER-5943