Add customisation help to docs

[vkbo]

Summary:

This PR adds instructions on how to customise novelWriter to the main documentation. This is added in a new section. Also a section of system file locations is added to the tech docs.

Related Issue(s):

Relates to, but doesn't fully resolve #982. See also Discussion #737.

Resolves #892.

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed