Add ecTrans documentation site

[samhatfield]

Addresses #113.

This PR will add the capability to build and publish to sites.ecmwf.int/docs/ectrans a documentation site.

The site is built primarily with FORD. This generates a hyperlinked code directory for src/trans. In addition it takes some hand-written Markdown files to produce a "User Guide" section.

There is an API section in the User Guide which is generated automatically by extracting a docblock from each of the files in src/trans/include/ectrans. This docblock is demarcated with begin_doc_block and end_doc_block tokens and should be written in Markdown. It will be copied verbatim into the api.md file (with Fortran comment markers stripped out. This is done by the build.sh script. Whether we actually need this is up for debate. I thought it would be useful so a first-time user doesn't need to wade into the code directory to find the usage for e.g. SETUP_TRANS. But with better commenting, the specific page for SETUP_TRANS has the potential to be just as useful. Perhaps the API page should just have a list of links to the corresponding pages for each external subroutine.

A subsite is also generated for transi using Doxygen.

TODO:

• Make sites.ecmwf.int/docs/ectrans public
• Finish User Guide: Introduction, Benchmarking, Interfacing with C, License, GPU offloading
• Add GitHub action for building and publishing docs automatically
• Rename docblock tokens to begin/end_docblock

I would say once the site is active and we have merged rgo-develop (and rebased this on top) we can merge this PR too, and gradually flesh it out.

[samhatfield]

https://sites.ecmwf.int/docs/ectrans/ now public.