Add script and GitHub action to automatically generate ReSpec versions of the spec (#2574)

* workflows: update used actions

* .

* bikeshed rendering (WIP)

* bikeshed; split into v2, v3 directories

* bikeshed; fixes to TOC,code blocks and indent levels

* bikeshed; work from unmodified .md files

* bikeshed; .gitignore

* bikeshed; separate header for v3.0.1 for now

* respec; as an alternative to bikeshed

* Rename bikeshed to md2html

* md2html; current setup

* md2html; wip

* md2html; fix 1 toc bug

* md2html; fix another toc bug

* md2html; fix 1 respec toc bug

* md2html; fix another bikeshed toc bug

* respec; former editor support - needs ReSpec 20.x

* Latest respec tweaks

* pre block css background, RFC ref fixes

* Github gist syntax highlighting css

* md2html: move to scripts dir

* md2html: resolve relative links for examples

* md2html: use respec js from spec.openapis.org site

* md2html: find gist.css regardless of run dir

* md2html: don't mix https and http content (toc)

* md2html; include google analytics

* md2html; build putative versions 4,5 etc

* md2html; tidying up

* md2html; specify lang="en" in html

* build: update deps

* fix: style examples bugs, refs #2488

* fix: update highlightjs call in md2html

* feat: add github action

Signed-off-by: Mike Ralphson <mike.ralphson@gmail.com>

* build: create a `latest.html` ReSpec version

Signed-off-by: Mike Ralphson <mike.ralphson@gmail.com>