Fixed environment docs for recent Ubuntu versions

[sethforprivacy]

Me and a teammate noticed that the build docs fail on more recent versions of Ubuntu, and I tracked it down to being due to OpenSSL version differences.

I've updated the environment doc to include detailed steps to resolving this issue with a pinned OpenSSL version that I have tested, as well as linked out to the source of those instructions.

Note that I also cleaned up the headers to be properly Markdown compliant.