Improve introduction into buildsystem

[YuriSizov]

Based on some discussions on RocketChat we've identified that some of the buildsystem documentation may be lacking in terms of introductory sections. So I've added an intro section to the Introduction to the buildsystem and also the its parent Building from source article. (cc @anvilfolk)

I also tries to improve the navigation a bit, by grouping related articles in the Engine development and Building from source articles. With all the changes it looks like this:

[nlupugla]

It'd be great if you could also suggest dev_mode=yes or whatever the syntax is for the preset that generates debug symbols and tests.

[nlupugla]

Related: would it be worth having a sort of "cheat sheet" and/or "FAQ" page? For example, compiledb=yes or scu are commands that can be super helpful, but are easy to miss by the time you have 10 tabs open. I know these things are all written in the docs, but it doesn't hurt to repeat important information a few times in different places.

[YuriSizov]

It'd be great if you could also suggest dev_mode=yes or whatever the syntax is for the preset that generates debug symbols and tests.

@nlupugla dev_build=yes generates debug symbols, but not tests. That's sufficient to develop the engine (that's what I use). The rest is already explained in the body of the article.

[mhilbrunner]

Thanks :) Merged!

[mhilbrunner]

Cherry-picked to 4.1