add mainpage for doxygen with link to other documentation

[iantaylor-NOAA]

What is the feature?

• Adds some text to the main doxygen page pointing to other documentation sources so it's not just a blank page from which it's hard to return to the rest of FIMS. This was discussed by the documentation group led by @Bai-Li-NOAA in July (notes here) but I had hoped for additional improvements. I now think that including this simple change in M2 is a good first step and the page can be improved further in future milestones.

How have you implemented the solution?

• Add a mainpage.dox file which will be compiled by the build-deploy-doxygen github action https://github.com/NOAA-FIMS/FIMS/actions/workflows/build-deploy-doxygen.yml although that will only run when dev is merged into main. The results of this PR are shown in the page on the right in screenshot below which was created by manually triggering the workflow on the doxygen-mainpage branch which temporarily replaced the doxygen page and then running it again on main to roll it back to the status-quo (left side of screenshot).
  

Does the PR impact any other area of the project, maybe another repo?

• No

[github-actions]

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[kellijohnson-NOAA]

Thanks @iantaylor-NOAA for this PR. I ❤️ it and would have never known to do this. I think it is a great addition to the doxygen website. I fixed the spelling of "documentation", changed the style to use "Javadoc-style" instead of "Qt-style", spelled out FIMS, added the standard @file and @copyright tags to the file, and complied the doxygen on my machine locally using the following code to ensure it rendered appropriately given the style changes.

rm -r build
cmake -S . -B build -G Ninja
cmake --build build --parallel 16

I will merge in once all the checks have passed.