[DOCS] Move Grok syntax docs to separate reference page

[jrodewig]

Currently, our Grok syntax docs reside in the Grok ingest processor docs.

Now that runtime fields also support Grok, we should consider moving those docs into a separate reference section.

Relates to #72580

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[lockewritesdocs]

This is also true for Dissect. Looking ahead, runtime field will start supporting other processing options, so we might need a new section that's part of our scripting docs.

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)