Clarify install docs (#167)

* Clarify install docs

- add prerequisites
- highlight build isolation

* No dollar signs in docs

* Add build dir setting

Co-authored-by: Alex Fikl <alexfikl@gmail.com>

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Co-authored-by: Andreas Klöckner <inform@tiker.net>
Co-authored-by: Alex Fikl <alexfikl@gmail.com>