MATTER-5943: Add customer documentation for project upgrades

[Michael Duggan (miduggan24)]

Description

2.9.0 release of Matter had subset of apps refactored to support project upgrades. with this, doccurator/2.9.0 branch had references to apps both pre and post project upgrades refactor.

also, in matter_extension repo, we have "Extending Base App Implementation" section in each individual app README file, with content being more or less the same for each app.

this PR does two things:

1. remove new / legacy architecture references and only focus on new app architecture (all apps refactored to support project upgrades with this release)
2. migrate "Extending Base App Implementation" into common docs location
   • associated matter_extension PR will be opened to remove duplicated content from each app README file.

Related Issue

MATTER-5943

Changes Made

• Add Extending Base App Implementation to custom-matter-device.md
• Remove new / legacy architecture headings, keep content only from new architecture sections

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

Additional Notes

n/a