Improvements to git page

What does this MR do?

This merge request cherry-picks the commits from !45935 (closed) (thanks @avron!) and layers on editorial work to bring https://docs.gitlab.com/ee/university/training/topics/env_setup.html closer to GitLab tone and style. I can't merge tone and style suggestions into a contributor's fork, so cherry-picking his commits into an MR will help move this work forward faster, with less frustrating back-and-forth on the contributor's part.

My editorial work here isn't a full editorial scrubdown - I would split this area of the page apart into subheadings - but I'm aware that this page is expected to be revamped / reconsidered in the near term. (cc @cnorris)

Thanks @terrichu for bringing this MR to my attention.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team