Add a safe development OAuth configuration matrix

[brudex]

Proposed labels: documentation, area:providers, good first issue

Context

Contributors can find per-provider OAuth code and configuration screens, but
there is no single safe reference that explains repository-defined field names,
local callback expectations, test approaches, and log-redaction requirements.

Suggested starting points

• CONTRIBUTING.md
• docs/architecture.md
• services/platform/*/oauth.js
• public/scripts/app/oauth-connect-configuration.client.controller.js

Scope

Create a contributor-facing configuration matrix for the providers already
implemented in the repository. Link to official provider documentation where
helpful, list only repository-defined field names and local prerequisites, and
describe credential-safe local tests and redaction practices.

Non-goals

• Do not create provider apps, request credentials, or add secrets to example
  files.
• Do not document bypasses for OAuth consent, callback validation, or provider
  policy.

Acceptance criteria

• Every currently supported provider has an entry with repository-defined
  settings, callback-origin prerequisites, safe local verification, and
  redaction guidance.
• Examples contain placeholders only; no real client IDs, secrets, tokens, or
  customer URLs are added.
• Provider claims and field names match the current OAuth implementations.
• The new document is linked from an appropriate contributor/development
  document.

Verification

1. Cross-check each matrix row against the corresponding OAuth module and UI
   configuration code.
2. Run npm test.
3. Run git diff --check and search the new documentation for accidental
   secret-like values before submitting it.