organize assignments for neuroimaging2 electrophysiological methods

As discussed with @Spaak and @britta-wstnr, I have reorganized the workshop section a bit to allow for multiple versions of neuroimaging2 (for now 2023/24, 2024/25) to coexist. Furthermore, I have copied the assignment for SR1-5 over from the MATLAB live scripts, and formatted them as markdown/jekyll documents.

The starting page for the course is now at https://www.fieldtriptoolbox.org/workshop/neuroimaging2-2425/, which is a hidden page, i.e., there are no links to it from elsewhere. There is a similar (also hidden) page for 2324.

The page text is in https://github.com/fieldtrip/website/tree/master/workshop. The images are in https://github.com/fieldtrip/website/tree/master/assets/img/workshop

On https://www.fieldtriptoolbox.org/development/guideline/website/ you can find syntax guidelines for formatting the markdown so that Jekyll will render it properly. Please note that colons `:` are not allowed in page titles.

Although the changes (so far) have been merged and are also on the actual website, I suggest we keep this issue open for now for follow-up discussion.