Do not shadow development commands

In this MR I'm updating the (local) development documentation.

I'm intentionally reversing some of the documentation concerning development commands shadowed by bdt dev counterparts (namely bdt dev clone and bdt dev install). In this commit, I do not deprecate any of those commands, but only discourage their use to newcomers. Rationale: less maintenance, and easier to find public fora to solve issues if the user is aware of what they are actually doing.

This update also reviews some of the English, and simplifies the TL;DR section on the development bit, which is quite long already for something that should be more like a cheat-sheet.