Add tags or categories

[MichaelCurrin]

For Jekyll site, add tags (multiple) or categories (multiple) for pages.

Then a list of tags or categories can be built from those that are used.

The page structure could be flat all in one directory then. Though that becomes hard to edit from a long list and also avoid collisions (a prefix is used for filenames which is a like a folder). So maybe don't do that part.