Expand documentation for adding external authentication providers

[katrinDY]

To add

https://docs.google.com/document/d/1iABitok5AC5V3WQyY4r_hxlE5IUpT9wC9agCacwA5Kk/edit#

Where to add

https://github.com/cortezaproject/corteza-docs/tree/2023.3.x/src/modules/integrator-guide/pages/authentication/external

TODO

• [ ] add a new section for the content
• [ ] add content
• [ ] add screenshots