pip install clean up items

[runleonarun]

Docs page to update

https://docs.getdbt.com/docs/core/pip-install#installing-prereleases

Overview

Here are a few things that might need to be cleaned up that I noticed when reviewing #6502

• Can we delete this FAQ if this content is covered in this page?
  • Before making the previous change, look at where this FAQ is referenced to make sure it works on those pages.
• The TOC to the right is wonky because of the FAQs. I suggest we downgrade the headings from H2 ## to H3 ### so they don't show up.
  • Before making the previous change, look at where those two FAQs are referenced to make sure it works on those pages.
• Starting with "Using Virtual environments," upgrade the H3 to H2
• Upgrade all other headings one level and upgrade "Activate your virtual environment" to an H2.

Originally posted by @runleonarun in #6502 (comment)

[nataliefiann]

Will work on this alongside this PR