docs(sp-dev-guide): add how-to guide for implementing custom CA support

[ValentinGerlach]

What this PR does / why we need it:

Adds a how-to guide for service provider developers on implementing custom CA bundle support. The guide walks through:

1. Adding a caBundleRef field (using corev1.ConfigMapKeySelector) to the ProviderConfig API
2. Resolving the CA bundle ConfigMap from the platform cluster at reconcile time
3. Using the bundle for outbound HTTPS connections from the controller and/or workloads deployed into MCPs/workload clusters
4. A note on re-enqueueing on ConfigMap changes

Also adds a cross-reference from the ProviderConfig guidelines page to the new guide.

Which issue(s) this PR fixes:
Fixes #47

Release note:

Add how-to guide for implementing custom CA bundle support in service providers