Make the OPT recomendation clearer

There are two ways of installing Pleroma (OTP and from source), but people seem unaware of that and the difference between them. Often they use a from source installations because they use the guide with the name of their distro and are confused when they hear there's another way of installing Pleroma.

• I added an explanation on all relevant from source installation guides (not the BSD ones since they don't have OTP releases) and linked to the OTP installation guide for those who prefer that.
• I added the same explanation on the two OTP pages (the "migration to OTP" and "installing OTP") and refered to the from source installation guides for those who prefer that.

I got this from a video I found on peertube: https://spectra.video/videos/watch/e029ebde-d43c-4515-b10c-00f8c9476b17 about Pleroma starts at 02:20 (they say nice things ;) ), the confusion about the install docs starts at 12:15.

You can see the build of these docs at https://pleroma-docs-example.ilja.space/