Update install documentation for Linux package installation method name

Problem to solve

Lots of documentation doesn't follow the style guide for installation methods naming: https://docs.gitlab.com/ee/development/documentation/styleguide/index.html#how-to-document-different-installation-methods.

Proposal

Update the documentation in doc/install directory to align with style guide for Linux package installation methods naming.

Source installation method references can also be updated if they're nearby, otherwise they can be done in a separate effort.

Other links/references

Relates to &10716 (closed).