Improve the documentation for switching to major release .Net version

[epananth]

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

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

[epananth]

@dkurepa has already written a elaborate wiki for regular maintenance https://dev.azure.com/dnceng/internal/_wiki/wikis/DNCEng%20Services%20Wiki/1005/Stage-DotNet-pipeline

we will probably need to update this with the new findings