User manual: structure

[Tom-Willemsen]

As discussed at group meeting, do something similar to ISISComputingGroup/ibex_developers_manual#5 but for the user manual

Edit: rendered view at https://isiscomputinggroup.github.io/ibex_user_manual/

[rerpha]

• this look funny https://isiscomputinggroup.github.io/ibex_user_manual/FAQ.html#when-i-load-script-i-get-an-error-complaining-about-unicodeescape fixed