Add section for setting contrib.sites values in production installation docs

[calvinrw]

Proposed Changes

• Add section to production.rst with instructions for setting site name and site domain in the contrib.sites framework. This was an early stumbling block for me when setting up my own wger instance, because the password reset email uses these values.

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• New python code has been linted with with flake8 (flake8 --config .github/linters/.flake8 ./wger)
  and isort (isort)
• Added yourself to AUTHORS.rst

Other questions

• Does this PR introduce a breaking change such as a database migration? (i.e.
  what changes might users need to make in their running application due to
  this PR?)
  No

[rolandgeider]

Thanks!