MATTER-3938: removed all mentions of SMG repo from documentation, pointing to upda…

[miduggan24]

…ted docs in CSA repo

Description

Some of the documentation still refers to the SMG repo, this PR changed these references to the equivalent documentation in the CSA repo to ensure the most up to date information.

Related Issue

Closes MATTER-3938

Changes Made

• sld251-matter-thread/build-flash-mad.md: Point to lighting app README in CSA repo instead of in SMG repo
• sld407-matter-ota/02-ota-software-update.md: Point to README for creating a Multi-Chip .ota file in the CSA repo instead of the SMG repo.
• sld407-matter-ota/03-firmware-upgrades.md: Changed instructions from cloning SMG repo to CSA repo and updated link to CSA repo rather than SMG repo.

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)

Additional Notes