Improve pnpm add <git remote URL> documentation

[MichalBryxi]

The problem

I wanted to install a package from a specific commit on my github fork. And I was a biiit confused around the current section of Install from git repository documentation.

I went to a deep(ish) dive through commits and patches/discussions to understand how things are and eventually landing on reading the tests.

The solution

So to prevent future generations needing to go through similar conundrum, I'd propose enhancing said part of documentation to:

• Create sections to make visual/logical distinction of individual parts.
• Spell out all major possible syntaxes.
• Add more explanation to individual parts of the URL like semver: and path:.
• Copy & paste examples directly from tests so that we're reasonably sure they are correct.
• Explain defaults.
• Explain common cases like monorepo & forks.