Include professional support information in documentation

[LeandervdBijl]

What does this PR do?

Added information about professional support through the OpenSTEF Support Providers Directory.

Closes #

Type of change

• Bug fix
• New feature
• Breaking change (see checklist below)
• Documentation
• Refactor / chore / CI

Breaking changes checklist

• Public API, config schema, or serialized/pickled objects changed in a way that affects existing users
• If yes: see the breaking changes guide for the pickle/_migrate_state migration pattern

Migration path for existing users (e.g. "old pickled XScaler objects auto-migrate on load", "users must now pass X explicitly"):

AI disclosure

• No AI assistance was used (beyond grammar/spelling)
• AI assistance was used — tool(s):
  • I have reviewed, understand, and can explain all AI-generated code in this PR
  • This is disclosed in a commit message (e.g. Assisted-by: <tool name>)

Checklist

• poe all --check passes locally
• Tests added/updated for the change
• Documentation updated (docstrings, user guide, examples) if needed
• Commits are signed off per our DCO (git commit -s)
• PR title follows Conventional Commits (e.g. feat: ..., fix: ..., feat!: ... for breaking changes)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud