document: https://github.com/keycloak/keycloak-documentation/commit/a4d4e54a39d8812ba5ba99a0fa08b59404617003

Backport upstream docs change with regards to "NullPointerException when confirming logout"

This is the commit:
https://github.com/keycloak/keycloak-documentation/commit/a4d4e54a39d8812ba5ba99a0fa08b59404617003

The tricky thing is, that commit includes also changes related to addition of `Valid Post Logout Redirect URIs` to client, which should NOT be included in the RHSSO 7.6.1 documentation as it was added just to the Keycloak main, but not to the RHSSO 7.6.1.

More details:

• The second paragraph from the file "19_0_0.adoc" should NOT be added to RHSSO 7.6.1 docs (It is the paragraph started with "Configuration option `Valid Post Logout Redirect URIs` added to the OIDC client.")
• The changes from the files "logout.adoc" and "con-oidc-auth-flows.adoc" should be included as is, but the change from the file "con-basic-settings.adoc" should NOT be included in RHSSO 7.6.1 docs