docs(external): improve proto codecs docs

[pront]

Summary

Added links and examples.

Change Type

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

Is this a breaking change?

• Yes
• No

How did you test this PR?

Does this PR include user facing changes?

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

Checklist

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References

[jszwedko]

Nice! I think this'll clear up a common question people ask about this codec.

[datadog-vectordotdev]

Datadog Report

Branch report: pront/chore-proto-docs
Commit report: 24b0d66
Test service: vector

✅ 0 Failed, 7 Passed, 0 Skipped, 25.44s Total Time

[pront]

Needs a rebase on #22223