Update documentation for virtualenv-based installation.

Review Request #13027 — Created May 9, 2023 and updated

Review Board

This updates all of our Review Board installation/upgrade documentation
to show steps on setting up and using a Python virtual environment. This
is required for systems using PEP 668, and is a good best practice for
others as well.

the Installation guide now covers the differences between Python Virtual
Environments and System Installs, and talks about the benefits of the
virtual environments.

The installation steps now provide tabs for both methods. We've removed
any instructions that result in installing native Python packages (such
as python3-svn) to help standardize these steps. This also applies to
the site creation and upgrade pages, as well as some repository/settings
pages that reference pip install.

To help keep this manageable, all installation steps are now listed
under a "Let's Begin" section, and are numbered, letting people know
where they are in the steps. This has been done with some other
sub-steps and in other pages. The result is a much cleaner flow,
especially with the presence of the tabs.

Built the docs and went through them in the new docs UI. Checked for
build errors, and tried checking for spelling errors.

Update documentation for virtualenv-based installation.
Description From Last Updated

Typo: disrtibutions -> distributions

Checks run (2 succeeded)
flake8 passed.
JSHint passed.
  2. Typo: disrtibutions -> distributions