docs on compiling: friendlier to first-time contributors

[Brambor]

Summary

None

Purpose of change

While helping a first-time contributor making PR #72153, some issues in docs became apparent. Not mentioning git led the poor soul to think there is no more to be done with git than to git clone so they got stuck at git push. It turned out, they didn't even make a fork.

I also saw some inconsistencies and things I thought could be improved (after my changes, it is subjectively better).

Describe the solution

Directly regarding the conversation with the first-time contributor

• Link to git usage (example git workflow) from any compiling tutorial where appropriate.
• Mention that fork != branch.

Furthermore

• Some text made clickable.
• remove a duplicate sentence
• remove trailing whitespaces
• Sometimes remove the Note: marker as I feel it is the core of the text.
• etc.

Describe alternatives you've considered

Also, look for mentions that CDDA with history is 3GB. Replace them with the fact it is 6GB today.

Testing

Compiled the .MD file locally into HTML and click the links.

Additional context