Describe SASL option for KafkaSource

[matzew]

Describe the change you'd like to see

I do not see any SASL related documentation in the kafka source docs:
https://knative.dev/development/developer/eventing/sources/kafka-source/

However, the Source does support SASL in various ways, and it should be documented.

Some comments on the relevant bits are in here:
knative-extensions/eventing-kafka#237 (comment)

Ideally we have some doc that describe the following section, for SASL:

...
net:
  sasl:
   enable: true
   user:
      secretKeyRef:
        name: strimzi-sasl-secret
        key: user
    password:
      secretKeyRef:
        name: strimzi-sasl-secret
        key: password
    saslType:
      secretKeyRef:
        name: strimzi-sasl-secret
        key: saslType
...

[github-actions]

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[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[jmcx]

Suggesting to reopen this issue, we've struggled to implement this given the lack of documentation. cc @odacremolbap