More Doxygen improvements

Description

1. Because Doxygen's parsing of (GitLab-flavoured) Markdown isn't great, use an external program (e.g. pandoc, markdown) to convert Markdown to HTML which is then directly included in the Doxygen docs. If a converter can't be found, Markdown is just included as verbatim text, so people can still build the documentation (e.g. to check for errors) without having to install another dependency. Because (GitLab-flavoured) Markdown is now never parsed by Doxygen, this will avoid silly errors like ba513ec4 in future. Of course for the online documentation pandoc will need to be installed into the relevant containers.
2. Scrape .gitlab/CODEOWNERS to get the maintainers for each package, as well as the general LALSuite maintainers. Add these names to the Authors page (now Authors/Maintainers).
3. Change the colour for unvisited hyperlinks so that they stand out more.
4. Some minor author list cleanups

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

Please provide details on any reviews related to this change and and the associated reviewers.