Improve the documentation for switching to major release .Net version

Whenever there is a major release a bunch of things have to be updated.

We have document that already covers what changes has to be made [here](https://dev.azure.com/dnceng/internal/_wiki/wikis/DNCEng%20Services%20Wiki/1005/Stage-DotNet-pipeline)

Eg:
Switching from eg: .Net 7 to .Net 8 involves
1) Update Scripts - errors and exceptions in scripting has to be updated
2) Documentation of this process - we already have a document in place, we just need to make sure it is updated

Also address like:
1) Publishing feeds not found. Did you add the channel to feeds.json and this script?
2) The property 'dotnet8' cannot be found on this object. Verify that the property exists -> [Link](https://dev.azure.com/dnceng/internal/_build/results?buildId=2130286&view=results)