Update documentation

[lucydot]

The documentation has a basic tutorial workflow. However there should be some FAQs added:

• how to adjust the fermi level and update CBM/VBM (especially useful when there is no occupancy data)
• how to specify a particular band/k-point when the one you want is automatically found

Also change structure of documentation so that inputs section is extended to cover various DFT calculators.

[lucydot]

After some recent PR's I should also:

• update API documentation for Datavasprun
• Update Jupyter notebook example of reading in vasprun.xml

[lucydot]

The documentation site is looking pretty tired in general. It would be nice to migrate this to mkdocs and mkdocstrings (for automatic documentation from docstrings a la sphinx).