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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