gemroff, gemroff(1): add proper summary and include summary in `-h` output
Makefile: properly clean up all testing artifacts
README: add note about de facto standard Ada compiler
gemroff(5): escape all Gemroff markup listed

When using man-db (which uses groff(1) under-the-hood), some Gemroff markup is
interpreted as lower-level roff markup and breaks rendering.  Issue did not
occur with the bespoke man/mandoc parser in mandoc(1).

Reported by dzwdz
Bump version to v11.prerelease
Bump version to 10
tests/run_tests: update copyright header
docs/*: rerender markdown versions of man pages
Docs: add note about links not being HTML escaped
Rendering.Render_HTML: escape entities in all text other than links

This properly escapes relevant HTML entities (normally just `<`, `>` and `&`;
but also `"` and `'` in non-link attributes).  Links may still break if they
include quotes, but now body text should never break if it incidentally
includes one of those characters.
Gemroff.Rendering.Render_{Gemtext,Textual}: Include prefix in blank paragraph breaks inside blockquotes
Bump version to v10.prototype
Bump version to 9
Gemroff.Rendering.Render_HTML: add value attribute to make footnotes have proper numbers
contrib/is_gemtext_gemroff.awk: check for incompatibilities relating to preformatted alt text
Gemroff.Parsing: Fix bug in gemtext mode when encountering preformatted alt text without space between it and the ```
Gemroff.Rendering.Render_HTML: wrap <img> in <a> to allow clicking image to open full-size
Gemroff.Configuring: much more clear error message when configuration file doesn't exist
Tests: update tests for new extension replacement
Gemroff.Configuring: fix bug from old replace-extensions code