Add accessibility statement to the documentation

[astroDimitrios]

Opening this to add the generic accessibility statement for generated documentation.
See https://github.com/MetOffice/common-fortran-template/issues/14 where I will add the statement first.

Note: In the template repo linked above the theme allows me to add the statement to the footer. Doing so with the Read the Docs theme would mean overiding the footer html and updating it ourselves. I think the easier option is to add the statement as a normal part of the toc / sidebar at the bottom.