docs: update docs on shell support and using packages (#19486)

Shell integration no longer requires setting `SPACK_ROOT`, so we can
simplify the documentation on it. The docs on shell support and using
packages are getting a bit old, and information on `spack load` (which
seems to be everyone's most common way of using packages) is hard to
find.

This PR simplifies the shell documentation to remove SPACK_ROOT, and also
moves some sections around for clearer organization.

- [x] make docs on sourcing setup scripts clearer and simpler

- [x] introduce `spack load` early in the basic usage guide instead of
      burying it in the module docs

- [x] clean up module docs so that spack module tcl loads comes later

- [x] be clear about the different ways to use packages so that the users
      can find the docs better.

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

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