Rename Omnibus and Source options in the docs
### Problem to solve
For over the years, we've been using the name Omnibus GitLab to refer to the all-in-one package that installs GitLab. While most of us know it internally, it's confusing to first timers and maybe others.
Some things to know:
- The name Omnibus comes from the software we're using ([Chef Omnibus](https://github.com/chef/omnibus)) to bundle all the different components GitLab uses into one singe package. Its description is:
> Easily create full-stack installers for your project across a variety of platforms.
- It is the most common [installation method](https://docs.gitlab.com/ee/install/README.html) and we have dedicated docs for it, you may know https://docs.gitlab.com/omnibus/README.html :)
- The project where the packages for the various Linux distros are built is located at https://gitlab.com/gitlab-org/omnibus-gitlab.
- After a package is built, it's uploaded to GitLab's package server https://packages.gitlab.com/gitlab.
- Although the list is not exhaustive, most of the bundled software can be found at https://docs.gitlab.com/omnibus/package-information/defaults.html.
- The [installation from source](https://docs.gitlab.com/ee/install/installation.html) was how GitLab can be installed by installing all the needed software manually, one by one. This is what the Omnibus package made easier.
In https://gitlab.com/gitlab-com/www-gitlab-com/-/merge_requests/48988, we updated the copy, and we no longer refer to Omnibus, instead, we say "all-in-one package". We then switched to "Official Linux package" https://gitlab.com/gitlab-com/www-gitlab-com/-/merge_requests/52125.
We should consider to do the same for the docs. I know we're heavily invested to the term Omnibus at that point, but maybe changing it is for the best?
### Proposal
As discussed in https://gitlab.com/gitlab-com/support/support-team-meta/-/issues/3489, we're renaming:
- `Omnibus` to `Linux package (Omnibus)`
- `Source` to `Self-compiled (Source)`
### Who can address the issue
The gitlab~10954568 team
### Other links/references
https://gitlab.com/gitlab-com/www-gitlab-com/-/merge_requests/48988
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