RBAC doc update after feature transition enterprise -> st2 OSS core

[arm4b]

RBAC functionality was integrated into the st2 core: StackStorm/st2#5086
Update RBAC documentation to remove any mentions of EWC/Enterprise and sync it up.

• https://docs.stackstorm.com/rbac.html
• https://github.com/StackStorm/st2docs/blob/master/docs/source/rbac.rst

Additionally, new instructions should indicate how to enable the rbac via st2.conf:
https://github.com/extremenetworks/ewc-installer/blob/4ac4c34073998c5ffd8e6d746b7e0cb9b3b5d346/scripts/bwc-installer-deb.sh#L201-L204

As well as show the recommended default RBAC definitions:
https://github.com/extremenetworks/ewc-installer/blob/4ac4c34073998c5ffd8e6d746b7e0cb9b3b5d346/scripts/bwc-installer-deb.sh#L206-L222

cc @hnanchahal