As decided in https://gitlab.com/gitlab-org/gitlab-ce/issues/53593

Fix links as discovered by the lint in the docs project

Introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30295

Partial Clone and Sparse Checkout are the native Git approach to enormous
repositories. We should document the state of these features and update as our
support and Git's support for these features improves.

Change code references to codeblocks so they can be copied
more easily, and add link to doc from git topic index. Also
applies suggested changes.

- Link new doc to and from other docs
- Add tbs section

Many code blocks are 4spaced, and they render in GitLab
without coloring as a result, even though they are
fenced with a language label. If in a list, other items
will render as being in a code block too, even if not
meant to. This fixes all these issues, and cleans up
minor white space issues in /topic, /university, /update
and /workflow docs.