Restructures existing page and adds improvements.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/56172

- Also makes other minor Markdown fixes that were near the main fixes.

This adds `project:, file:, ref:` specification support.

This rewrites a syntax to allow include of templates.
This also normalises the syntax used by include: feature

gitlab-org/gitlab-shell!166 added support for collecting push options
from the environment, and passing them along to the
/internal/post_receive API endpoint.

This change handles the new push_options JSON element in the payload,
and passes them on through to the GitPushService and GitTagPushService
services.

Futhermore, it adds support for the first push option, ci.skip.  With
this change, one can use 'git push -o ci.skip' to skip CI pipe
execution. Note that the pipeline is still created, but in the "skipped"
state, just like with the 'ci skip' commit message text.

Implements #18667

Kudos to @eread for feedback!

- Split up refs and kubernetes to their own section
- Use only and except in all headings explicitly
- Document the boolean logic of only and except

This is a documentation change about #15310.

Signed-off-by: NTakuya Noguchi <takninnovationresearch@gmail.com>

This prevents some of the abusive behaviors, of someone putting 100000 and creating out of memory condition easily