Clean up about page

[mhkuu]

And add links and documentation to e.g.:

• (new) user manual
• (new) technical documentation
• github page
• query syntax Elasticsearch
• spelling Marchant (e.g. https://nl.wikipedia.org/wiki/Geschiedenis_van_de_Nederlandse_spelling)

Also, this could/should probably be static HTML instead of something generated by Dojo :-)