* [DOCS] Add docs for runtime fields (#62653)
* First steps in docs for runtime fields.
* Adding new page for runtime fields.
* Adding page for runtime fields.
* Adding more to the runtime fields topic.
* Adding parameters and retrieval options for runtime fields.
* Adding TESTSETUP for index creation.
* Incorporating review feedback.
* Incorporating reviewer feedback.
* Adding examples for runtime fields.
* Adding more context and simplifying the example.
* Changing timestamp to @timestamp throughout.
* Removing duplicate @timestamp field.
* Expanding example to hopefully fix CI builds.
* Adding skip test for result.
* Adding missing callout.
* Adding TESTRESPONSEs, which are currently broken.
* Fixing TESTRESPONSEs.
* Incorporating review feedback.
* Several clarifications, better test cases, and other changes.
* Adding missing callout in example.
* Adding substitutions to TESTRESPONSE for shorter results shown.
* Shuffling some information and adding link to script-fields.
* Fixing typo.
* Updates for API redesign -- will break builds.
* Updating examples and including info about overriding fields.
* Updating examples.
* Adding info for using runtime fields in the search request.
* Adding that queries against runtime fields are expensive.
* Incorporating feedback from reviewers.
* Minor changes from reviews.
* Adding alias for test case.
* Adding aliases to PUT example.
* Fixing test cases, for real this time.
* Updating use cases and introducing overlay throughout.
* Edits, adding 'shadowing', and explaining shadowing better.
* Streamlining tests and other changes.
* Fix formatting in example for test.
* Apply suggestions from code review
* Incorporating reviewer feedback 7 Dec
* Shifting structure of mapping page to fix cross links.
* Revisions for shadowing, overview, and other sections.
* Removing dot notation section and incorporating review changes.
* Adding updated example for shadowing.
* Streamlining shadowing example and TESTRESPONSEs.
* Adding type field in responses for 7.x to fix test cases.
