Kibana updates, remove links from JSON and split is-null/is-not-null (#125986) (#126039)

In particular:
* Remove all links (both asciidoc and markdown) from the JSON definition files.
  * This required a two phase edit, from asciidoc links to markdown, and then removal of markdown (replace with markdown text). This is because the asciidoc does not have the display text, and because some links were already markdown.
* Split predicates into is_null and is_not_null
  * We kept the old combined version because the main docs still use that, so now we have both combined and separate versions, and Kibana can select the version they want.

Author: craigtaverner

docs/reference/query-languages/esql/_snippets/operators/examples/is_not_null.md

@@ -0,0 +1,15 @@
+% This is generated by ESQL's AbstractFunctionTestCase. Do no edit it. See ../README.md for how to regenerate it.
+
+**Example**
+
+```esql
+FROM employees
+| WHERE is_rehired IS NOT NULL
+| STATS COUNT(emp_no)
+```
+
+| COUNT(emp_no):long |
+| --- |
+| 84 |
+
+

docs/reference/query-languages/esql/_snippets/operators/examples/is_null.md

@@ -0,0 +1,19 @@
+% This is generated by ESQL's AbstractFunctionTestCase. Do no edit it. See ../README.md for how to regenerate it.
+
+**Example**
+
+```esql
+FROM employees
+| WHERE birth_date IS NULL
+| KEEP first_name, last_name
+| SORT first_name
+| LIMIT 3
+```
+
+| first_name:keyword | last_name:keyword |
+| --- | --- |
+| Basil | Tramer |
+| Florian | Syrotiuk |
+| Lucien | Rosenbaum |
+
+

docs/reference/query-languages/esql/_snippets/operators/types/is_not_null.md

@@ -0,0 +1,22 @@
+% This is generated by ESQL's AbstractFunctionTestCase. Do no edit it. See ../README.md for how to regenerate it.
+
+**Supported types**
+
+| field | result |
+| --- | --- |
+| boolean | boolean |
+| cartesian_point | boolean |
+| cartesian_shape | boolean |
+| date | boolean |
+| date_nanos | boolean |
+| double | boolean |
+| geo_point | boolean |
+| geo_shape | boolean |
+| integer | boolean |
+| ip | boolean |
+| keyword | boolean |
+| long | boolean |
+| text | boolean |
+| unsigned_long | boolean |
+| version | boolean |
+

docs/reference/query-languages/esql/_snippets/operators/types/is_null.md

@@ -0,0 +1,22 @@
+% This is generated by ESQL's AbstractFunctionTestCase. Do no edit it. See ../README.md for how to regenerate it.
+
+**Supported types**
+
+| field | result |
+| --- | --- |
+| boolean | boolean |
+| cartesian_point | boolean |
+| cartesian_shape | boolean |
+| date | boolean |
+| date_nanos | boolean |
+| double | boolean |
+| geo_point | boolean |
+| geo_shape | boolean |
+| integer | boolean |
+| ip | boolean |
+| keyword | boolean |
+| long | boolean |
+| text | boolean |
+| unsigned_long | boolean |
+| version | boolean |
+