docs(kafka source, kafka sink): include kafka source in `librdkafka` section

[tessneau]

Summary

In the docs for the kafka source, the section about librdkafka here currently only mentions the sink. This includes the source in the description since we also use it there -- see rdkafka import in the source here.

Change Type

• [ ] Bug fix
• [ ] New feature
• [x] Non-functional (chore, refactoring, docs)
• [ ] Performance

Is this a breaking change?

• [ ] Yes
• [x] No

How did you test this PR?

• ran ./website/scripts/cue.sh fmt
• CI=true make check-docs validated succesfully

Does this PR include user facing changes?

• [ ] Yes. Please add a changelog fragment based on our guidelines.
• [x] No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• [x] Please read our Vector contributor resources.
• [n/a] If this PR introduces changes Vector dependencies (modifies Cargo.lock), please run dd-rust-license-tool write to regenerate the license inventory and commit the changes (if any). More details here.

References

[datadog-vectordotdev[bot]]

Datadog Report

Branch report: tess.neau/minor-kafka-source-doc-update Commit report: c74d615 Test service: vector

:white_check_mark: 0 Failed, 7 Passed, 0 Skipped, 25.46s Total Time