Add accessibility guidance

I've tried to explain the background of this briefly.

Just to note, I have been trying to contact the OSC about the possibility of publishing HTML exports from LaTeX, but have been ignored. Unfortunately, this seems to show that they see accessibility mainly as a box ticking exercise given that the tools for actual accessibility verification (e.g. screenreader emulation and automated checklist verification) of tagged PDFs are a lot worse, commmerical or at least closed source, and Windows only versus HTML where there's obviously a lot more resources.

Given this is the only option left to us, I still think it's better to attempt to produce tagged PDFs, since it may result in a better experience in case there are any disabled people wanting to read our theses.

I've just machine translated into Finnish in the README. Perhaps you can fix it up again?

The whole of accessibility.tex is untranslated. Is it better to make this document bilingual or have two? The reason I have made this a separate document is that it cannot be built on older versions of LaTeX since it uses experimental features.

My proposal is to target this experimental version of the LaTeX tagged PDF project for now, and then update the guidance next when the project is declared "finished".