Edit: auth-flow documentation for multi-client, backchannel logout, and JWKS

[amountainram]

Description

Changes

auth-flow.mdx (overview)

• Added multi-client support, back-channel logout, and JWKS endpoint to TL;DR
• New Back-Channel Logout section with sequence diagram showing the server-to-server session termination flow
• New Multi-Client Support section explaining shared session cookie with per-client token storage and ?client_id routing

configuration.mdx

• New Multi-Client Configuration section with single-client and multi-client JSON config examples
• New Envoy Routing for Multi-Client section with three routing techniques: virtual hosts (domain-based), listener-based, and path-prefix discrimination — plus client-id-inject.lua helper
• New Back-Channel Logout section with AS settings table (backchannel logout URL, JWKS URL)
• Added Envoy endpoint routes for /oidc/logout/backchannel and /.well-known/jwks.json

frontend-application.mdx

• Added Multi-Client Note explaining that client_id must be injected at the infrastructure layer

static/schemas

• Added latest authtool_bff.schema.json (updated with multi-client clients array support)

Pull Request Type

• Documentation content changes
• Bugfix / Missing Redirects
• Docusaurus site code changes

PR Checklist

• The commit message follows our guidelines included in the CONTRIBUTING.md
• All tests of Lint, cspell and check-content pass. (How to launch them?)
• No sensitive content has been committed