Our installation instructions hadn't kept up with changes to various

Linux distributions. Package names have changed, Python packages have

changed, and our complete set of alternative options weren't up-to-date.
This change completely reworks the installation instructions.
The Linux instructions now start off by introducing the user to the

guide, providing alternative means of installation (Docker and

RBCommons), and providing a support link. It no longer lists Bitnami,

which no longer provides Review Board VMs or installers.
It then has a section fully about compatibility containing:


The Python versions needed (which is automatically set based on our

  minimum version constant)


The Linux distributions the instructions were tested with (mirroring

  our automated testing of installation steps we have elsewhere).


The list of supported databases (and the recommended versions)


The web servers tested.


Then it has a step about preparing for installation, covering the fact

that admin access is needed and talking about how to set up HTTP(S)

proxies.
Finally, the installation steps. These have been cleaned up a bit,

better differentiating required vs. optional steps, and removing a lot

of extra text that wasn't really needed. They use the latest package

names for our dependencies.
There are new sections for installing:


Power Pack


Authentication backends


CDN support (this already existed for S3 and Swift, but are now

  grouped into a container section)


The final step talks about what a site directory is, and leads the user

to the page for creating one.
These new instructions should be much easier to follow and should

provide a better initial experience.
The Windows docs have been replaced with some new content on installing

in WSL, or using Docker or RBCommons.

Verified all the steps against our automated install tests.
Went through and read each paragraph for clarity (though this

could use another pair of eyes).
Checked for spelling errors, bad links, and build errors.