Update Powerline docs

[JanDeDobbeleer]

A few changes:

1. There were a few comments from people getting confused by the -RequiredVersion 2.0.412 setting to install oh-my-posh. As that's probably a leftover from a time long passed I removed it.
2. There's a mention of golang as a requirement to install oh-my-posh on WSL, that's incorrect and a leftover from powerline-go instructions.
3. To guide users to an easier installation/update experience, I added an info block with this context pointing to the oh-my-posh docs.

As a general question, I wanted to point to a relative URL in point 3, but I have no idea which one to use. If someone can point me in the right direction, would be appreciated!

relates to #420, #413, #404, #382, #370, #369, #364, #363 (the list goes on 😓 )

[opbld32]

Docs Build status updates of commit 98bf80e:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
TerminalDocs/tutorials/powerline-setup.md	⚠️Warning	View	Details

TerminalDocs/tutorials/powerline-setup.md

• Line 107, Column 102: [Warning-hard-coded-locale] Link 'https://docs.microsoft.com/en-us/windows/terminal/tutorials/powerline-setup#customize-your-ubuntu-prompt' contains locale code 'en-us'. For localizability, remove 'en-us' from links to most Microsoft sites.
• Line 107, Column 102: [Suggestion-docs-link-absolute] Absolute link 'https://docs.microsoft.com/en-us/windows/terminal/tutorials/powerline-setup#customize-your-ubuntu-prompt' will be broken in isolated environments. Replace with a relative link.

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[mattwojo]

Hey @JanDeDobbeleer - Thanks for filing this PR. We overhauled the PowerLine / Oh My Posh content in our docs and pointed to your OMP docs quite a bit more. Just did another update - if you have a spare moment to take a look, here's the PR: #467

Thanks for your help here -- OMP is amazing and hopefully we'll get folks on the right path to add some awesome custom themes without the confusion our old content included.