docs: Document how to install the authd-oidc broker

[adombeck]

Document how to install the generic OIDC broker.

UDENG-8818

[edibotopic]

This looks fine. A few questions:

• When will the snap be available to install?
• Will the generic broker require an example configuration with an OIDC provider?
• Should we include the oidc broker on the homepage (in the tabs set), with a link to these installation guidelines?

PR #1200 should fix the docs build issue.

[adombeck]

When will the snap be available to install?

The first development release to the edge channel will be available once the Snap store team approved it (it's pending review since earlier this week).

I hope to be able to do the first stable release with the authd + broker release scheduled for next pulse.

Will the generic broker require an example configuration with an OIDC provider?
Should we include the oidc broker on the homepage (in the tabs set), with a link to these installation guidelines?

The plan is to document how to use Keycloak with the generic OIDC broker (https://warthogs.atlassian.net/browse/UDENG-8809). I would do that in a "Keycloak" tab set in configure-authd.md.

We're also considering documenting how to use Okta with the generic OIDC broker (#1197).

[edibotopic]

Excellent @adombeck --- all sounds fantastic. I will approve this one now after those clarifications.