Add instructions to replicate the ioos-documentation-jekyll-skeleton with a fresh upstream Jekyll theme

[mwengren]

Documentation update: It would be nice to add a new doc page alongside the existing HOWTO docs that describes steps for a user to fork this template and replace the submodule elements with a freshly-forked copy of the tomjoht/documentation-theme-jekyll Jekyll theme.

It would be rather complicated however, something like:

1. fork this repo
2. delete the _data/navbars submodule, delete the theme submodule
3. fork upstream tomjoht/documentation-theme-jekyll Jekyll theme
4. copy navbars branch from our own theme to the forked Jekyll theme
5. initialize _data/navbars submodule with link to newly created navbars branch of forked Jekyll theme
6. initialize theme submodule with link to newly main branch of forked Jekyll theme
7. edit files in forked Jekyll theme main branch to work with modified resources paths under _data directory and theme submodule directory, and other necessary mods for the submodule Jekyll theme approach to work. A few examples for reference: clean up orphaned branches documentation-theme-jekyll#3 (comment)
8. deploy forked skeleton repo to GitHub Pages and see what happens

This may be too complicated for anyone to really want to do, but at least we have some steps documenting roughly how this repo was created for someone to attempt to replicate it if they like.