Process the remaining few subheadings for "create a merge request"
for accuracy.

Don't reference the pinned versions of "Merge requests" or "Issues"
even though it's visible. Use the long version.

I'm not the entire way through the page, but I'm losing steam and
we can get value from the updates I've done so far. This commit steps
through the first few subheadings on this page, checking each step
of each process, and updating them to match current expectations.

I can't convince others to add a test, so I should fix these
instances of the word 'may.' It never registers on me as an
issue.

Update the links in all user and admin pages

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

Because the left nav is changing
and will have a button instead
of carat, and no magnifying glass.

Related to: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/127935

Because we changed the way we
do the markdown.

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

To close off an old, old problem, let's describe the inheritance
of merge request templates.

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.

We never documented how to configure branch naming defaults for when
users create branches from issues. This required shuffling some
existing content around on the branches page, and documenting some
old OLD tech debt - namely how to create branches, create a branch
in an empty project, and an overarching branch-naming section.

The disqus integration was removed from the docs site,
so these identifiers don't do anything now.

Combine these sections down into one section that adheres to our
content style expectations. Remove several painfully old images. Flow
text back inline for ease of understanding.

Fix instances of naming UI elements that go against our style guide.
Add in hard line wraps. This work should be more fully documented
but let's start here.

For users who need this level of help, we should tell them how to
upload a file.

Flow the information into the main text.
Tighten the language in the cross-links.
Punctuate list items consistently.

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

Because the UI changed.
Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/366082/

Page edits and link updates

Changelog: changed