Continue chipping away at the list of broken or redirecting links.

Start somewhere in the list. Begin fixing links that are broken
or that redirect.

Because for now, we're changing the format.

Related to: https://gitlab.com/gitlab-org/gitlab-docs/-/issues/1751

The UI now says "Code" so let's make the docs match reality.

Update the links in all docs except user and admin pages

Because the page felt cluttered to me
Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/425355

This is not used on the current docs website
and needs to be removed for future compatibility
with Hugo.

Because these concepts were duplicates
I put them in a get started page instead
And redirected the Concepts page.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/425355

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/432764

**Problem**

The current version of the documentation is confusing. It might be read
as user needs to provide their GitLab username instead of their name in
git configuration.

**Solution**

Replace `username` with `full name` in the documentation

As part of working on clone instructions in another repo, I needed
to link to these instructions. I realized they needed our current
starting-point info, and needed clearer instructions on where to
find the Clone button.

Because we changed the way we
do the markdown.

Related to: https://gitlab.com/gitlab-org/gitlab-docs/-/issues/1690

Add link to git-credential-oauth

Describe the actual benefits above and beyond the Git rules.

Bring subheadings more in tune with expected CTRT standards.
Rename subheadings to use present tense instead of gerunds.
Clean up a lot of gunky old wording.

As part of this work, I realized an important part of the forking
instructions were captured on a more generic advanced-settings page.
I've pulled that information back into this page, where it belongs.
That page also had some forking-related troubleshooting info that
also should have been here. I reunited that troubleshooting info
here as well.

Bring titles more in line with what's displayed in the left nav.

Because under Use GitLab, things
are not uniform or consistent.
Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/384502

Our style guidelines say not to use
https://docs.gitlab.com/ee/development/documentation/styleguide/word_list.html\#eg

Also deletes one unused image

This site has moved from docs.microsoft.com to learn.microsoft.com.
Rather than dealing with these individually, let's find-and-replace
to fix them all at once.

Related epic: https://gitlab.com/groups/gitlab-com/-/epics/1940

Related to group: Source Code

This deals with how users can create and SSH
keys to their account, so it should be in
user/profile, not a top level directory by itself.

To better incorporate the new Git
tutorial, this MR is meant to help
organize the existing Git content.
It's a small MVC that helps move
in a different direction and remove
duplicate content