Add tutorial for adding custom resource handlers

[MiCurry]

This PR adds a new page in the tutorial section of the docs that details how to add a custom resource handler.

Currently, it contains a section on adding resource handlers to one-file, standalone builds with Nuitka and PyInstaller. On the Nuitka and PyInstaller pages a references is added to this section.

This PR also fixes a small typo on the PyInstaller Tutorial page.

This PR would close #1200.

[MiCurry]

@Cleptomania thanks for the comments. I've added a commit that addresses them. Let me know what you feel about the added verbiage on the multi-directory resource handler section!