Gemroff v5

Major new feature: the ".Fn" macro for textual footnotes.  Inserts a footnote
at the designated footnote-position and then adds a ref link.  Like an inline
link in non-HTML formats, and in fact is mixed alongside the links' footnotes.

- Gemroff.Main: update version number, erroneous v0.0.0
- Gemroff.Rendering: fix unclear Program_Error raise
- Gemroff.Rendering.Gemtext: fix bug when preformatted open doesn't have alt text
- Gemroff.Rendering: remove some code duplication
- Gemroff.*: Implement textual footnotes
- Tests: test textual footnotes
- contrib *: support textual footnotes
- docs: add documentation for .Fn and regenerate markdown
- Gemroff: fix bug, make Footnotes be Inline_Types
- *: update copyright headers for modified files
- Release v5