documentation: build-system phases + build-time tests (#2780)

* documentation: reworked packaging guide to add build-system phases

* documentation: improvements to AutotoolsPackage autodocs

* build_systems: updated autodocs

* run-tests: added a few information on how to run tests fixes #2606 fixes#2605

* documentation: fixed items brought up by @davydden

    * typos in docs
    * consistent use of 'build system' (i.e. removed 'build-system' from docs)
    * added a note on possible default implementations for build-time tests

* documentation: fixed items brought up by @citibeth

    * added note to explain the difference between build system and language used in a package
    * capitalized bullet items
    * added link to API docs

* documentation: fixed multiple cross-references after rebase

* documentation: fixed minor issues raised by @tgamblin

* documentation: added entry in table for the `PythonPackage` class

* docs: fixed issues brought up by @citybeth in the second review