QGIS documentation format

[shyamamrith]

See: https://docs.qgis.org/3.28/en/docs/pyqgis_developer_cookbook/plugins/plugins.html#documenting-plugins

• [x] Check if there are any specific format requirements
• [x] Complete documentation for external use - i.e. make general, remove installation part...
• [x] Host online - probably requires markdown or rst format
• [x] Add link to documentation website in plugin help menu, see: https://docs.qgis.org/3.28/en/docs/pyqgis_developer_cookbook/plugins/plugins.html#mainplugin-py

[shyamamrith]

Docs moved here: https://github.com/UCL/simstock/blob/main/docs/source/simstockqgis.rst