doc: initial support for pdf export at doc. generation

- custom cover page for pdf
 - Center and resize logo
 - Place slogan just below logo
 - Ensure all text on cover page is in matching grey

Please note, the mkdocs-to-pdf plugin works, but it has a bug in the
enumeration of sections.  We may need to fork it to make it work the
way users expect.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/docs.yml

@@ -46,6 +46,7 @@ jobs:
           pipx inject mkdocs pymdown-extensions
           pipx inject mkdocs mkdocs-callouts
           pipx inject mkdocs mike
+          pipx inject mkdocs mkdocs-to-pdf
           # Workaround, if pipx inject fails to install symlink
           ln -s "$(pipx environment -V PIPX_LOCAL_VENVS)/mkdocs/bin/mike" \
                 "$(pipx environment -V PIPX_BIN_DIR)/mike" || true