Add how-to guide for HAProxy to use self-signed certificates to serve requests

[weiiwang01]

What this PR does

Adds a new how-to guide with two ways to serve HTTPS using self-signed certificates in haproxy.

Why we need it

Checklist

• I followed the contributing guide
• I added or updated the documentation (if applicable)
• I updated docs/changelog.md with user-relevant changes
• I added a change artifact for user-relevant changes in docs/release-notes/artifacts. If no change artifact is necessary, I tagged the PR with the label no-release-note.
• I used AI to assist with preparing this PR
• I added or updated tests as needed (unit and integration)
• If integration test modules are used: I updated the workflow configuration
  (e.g., in .github/workflows/integration_tests.yaml, ensure the modules list is correct)
• If this PR involves a Grafana dashboard: I added a screenshot of the dashboard
• If this PR involves Terraform: terraform fmt passes and tflint reports no errors

[erinecon]

Thank you so much for writing this how-to guide!

[erinecon]

Why is pollen being deployed here?

[weiiwang01]

We need a backend application to trigger the haproxy charm to request a certification. Pollen is used for that backend application, similar to the haproxy tutorial.

[erinecon]

Ah okay, so if I'm understanding correctly, in practice the user would have their own backend app that requires a certificate, and we're using Pollen as an example/stand-in?

[erinecon]

Thanks for incorporating the first round of feedback! The content looks good overall, just a few more nits to clarify the role that the Pollen charm plays in the guide.

[erinecon]

Ah okay, so if I'm understanding correctly, in practice the user would have their own backend app that requires a certificate, and we're using Pollen as an example/stand-in?

[erinecon]

If I'm understanding the role Pollen is playing here, then I think we should highlight that explicitly, maybe something like:

Suggested change

	To use the self-signed-certificates charm with the HAProxy charm, simply deploy
	the self-signed-certificates charm and integrate it with the HAProxy charm using
	the `certificates` relation.
	To use the self-signed-certificates charm with the HAProxy charm, simply deploy
	the self-signed-certificates charm and integrate it with the HAProxy charm using
	the `certificates` relation. This guide uses the Pollen charm as the backend application
	requiring a certificate.

[erinecon]

If I'm understanding the role Pollen is playing here, I think we should explicitly state the role in text, maybe something like:

Suggested change

	manual-tls-certificates charm and integrate it with the HAProxy charm using
	the `certificates` relation.
	manual-tls-certificates charm and integrate it with the HAProxy charm using
	the `certificates` relation. This guide uses the Pollen charm as the backend application
	requiring a certificate.