Adding a Keycloak High Availability section to Keycloak's docs

[ahus1]

Merge this only after:

• Adding a Keycloak High Availability section to Keycloak's docs keycloak#24913 has been merged
• Keycloak 23 has been released

Related to keycloak/keycloak#24844

[ahus1]

To build the Keycloak web site locally, first build PR keycloak/keycloak#24844 locally.

To check out this PR locally in a branch named pr-433-review, assuming that the Keycloak upstream repository is named upstream.

git fetch upstream pull/433/head:pr-433-review
git checkout pr-433-review

You're free to adjust the name pr-433-review to match your local conventions and change it both commands above.

To build the Keycloak website locally, run

mvn clean install -Dversion.keycloak=999.0.0-SNAPSHOT

Afterwards, open the file target/web/index.html in a local browser for previewing all guides of the HA group.

A more general guide is available in README.md