chore(external docs): clarify ElasticSearch bulk actions #7616

[Shogobg]

In sinks.cue, the only_fields and except_fields have the same description. This change fixes the description to show the correct function.

In elasticsearch.cue, i've added a clarification that the operation will not update the document, but replace it. Usually updates will "patch" a partial document, but the index action replaces the full document, so if a partial document is sent, it will overwrite the existing one.

Fix a broken link in CONTRIBUTING.md

[zsherman]

Thank you!

[jszwedko]

Awesome, thank you @Shogobg !

[jszwedko]

@Shogobg thanks for the contribution! If you get a second, would you mind signing off the commit for DCO?

It should just be something like:

git commit --amend --no-edit --signoff
git push --force-with-lease origin #7616-Improve-documentation

You can see https://github.com/timberio/vector/blob/master/CONTRIBUTING.md#signing-off for more details.

[Shogobg]

@jszwedko I have updated the commit to sign it off.

[jszwedko]

Thanks @Shogobg !