Simplify interaction with BIDS schema auto build script

[sappelhoff]

I think the entry in the docs on using the bids_schema.py file is wrong:

python tools/bids_schema.py entity src/schema/ src/99-appendices/04-entity-table.md

"entity" is not a valid call:

usage: bids [-h] {show,entitytable,entities} ...
bids: error: invalid choice: 'entity' (choose from 'show', 'entitytable', 'entities')

Furthermore, can we perhaps simplify the command, so that users preferably only have to call something like rebuild_bids_schema or something like that and then entity table and entities is automatically taken care of @tsalo ?

I noticed that there were some issues with the usage in #690 and then proceeded to repeat some mistakes until I (hopefully?) got it right.

[sappelhoff]

this was closed as part of #610