Website changes for MFEM v4.8

[tzanio]

🔗 Companion PR for the mfem-4.8 release in mfem/mfem#4681

🔴 TODO:

• Merge Add documentation on the "Building MFEM" page for CMake install #291?
• Merge Add ex36 tutorial #305?
• Update list of contributors ✔️
• Update publications ✔️
• Make sure all new examples and miniapps are documented
• Add new items to the Gallery?
• Update the size and SLOC for this version in download.md
• Update doxygen docs
• Check the links to doxygen, e.g. in features.md

🟢 DONE:

• Update version number in index.md, download.md and news.md
• Create bit.ly shortlink: bit.ly/mfem-4-8 now points to mfem.github.io/releases/mfem-4.8.tgz
• Note BoundaryMassIntegrator for DG Robin BCs #307

[brendankeith]

@tzanio I will get back to #305 (as well as mfem/mfem#4460) at the end of the week

[tzanio]

@adam-sim-dev, we would like to add you to the list of MFEM contributors in about.md but we usually list the full names.

If you are open to this, can you let us know your name, and also provide your prefered link (by default it will be https://github.com/adam-sim-dev)? (If not, no worries)