docs: Add keycloak PKCE and argo-cd cli method documentation

[albundy83]

• Add doc to configure keycloak and argo-cd with PKCE, the only way to be able to authenticate to argo-cd with keycloak using web ui AND argo-cd command line.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

Fixes [ISSUE #12124]
Fixes [ISSUE #10029]

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[albundy83]

To understand the need on this issue, I could mention this issue #12124 and also this one #10029.
My aim is to simplfy the way to correctly configure keycloak when you want to authenticate also using cli.