Fixes #668 - Generate and publish Doxygen

[jiridanek]

This will create a link in the right-hand panel of the GitHub project page, like the one I now have at my fork https://github.com/jiridanek/skupper-router

When you click the environment link, you get a list of environments, and you can open the latest version of the Doxygen documentation from there

The Doxygen is hosted at https://jiridanek.github.io/skupper-router (it will be at skupperproject.github.io for this repository, after PR is merged)

The site is updated when CI runs for the main branch. When running PR branches, the pages are not deployed.

[codecov-commenter]

Codecov Report

Merging #673 (b660442) into main (2aa053b) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #673      +/-   ##
==========================================
- Coverage   25.91%   25.90%   -0.01%     
==========================================
  Files         128      128              
  Lines       31195    31195              
  Branches     4973     4973              
==========================================
- Hits         8083     8081       -2     
  Misses      22059    22059              
- Partials     1053     1055       +2     

Flag	Coverage Δ
unittests	25.90% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/router_core/address_watch.c	73.56% <0.00%> (-1.15%)	⬇️
src/router_core/route_tables.c	16.34% <0.00%> (-0.28%)	⬇️

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[jiridanek]

An example of a fairly complete doxygen page is https://jiridanek.github.io/skupper-router/group__iterator.html

• I'm looking into removing the qd/qdr prefixes for purposes of function index generation now.

[jiridanek]

In order to make this actually useful

• remove or rewrite that outdated "experimental Qpid Dispatch Library" text
• render the markdown documents under https://github.com/skupperproject/skupper-router/tree/main/docs with this

The usefulness should come from

1. ensuring that code cross-references are valid, for example, if docs document talks about C function that was renamed in code, doc build will fail, which means that doc needs to be either updated, or deleted (if it is deemed to be too much of a burden to maintain); note that https://github.com/skupperproject/skupper-router/tree/main/decisions must not be crosschecked like this, because decisions are supposed to go out-of-date as time goes
   • edit: whoops, most files in /docs/ are in AsciiDoc, not Markdown, so Doxygen won't import them ;(
2. CI-ing doc comments in code, ensuring that Doxygen syntax is valid, whenever PR author uses some Doxygen
3. promote writing more Doxygen comments in the future

Overall, the GitHub markdown rendering capabilities (what it does on the fly) are quite sufficient for purposes of the /docs/ folder and nobody as of yet missed doxygen much