Added insert action changelog (#1886)

## Checklist

- [/] I have added
[tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
- [x] I have updated the
[docs](https://github.com/cursorless-dev/cursorless/tree/main/docs) and
[cheatsheet](https://github.com/cursorless-dev/cursorless/tree/main/cursorless-talon/src/cheatsheet)
- [/] I have not broken the cheatsheet

---------

Co-authored-by: Pokey Rule <[email protected]>

Author: AndreasArvidsson

changelog/2023-09-addedInsertPythonAction.md

@@ -0,0 +1,7 @@
+---
+tags: [enhancement, talon]
+mergeDate: 2023-09-10
+pullRequest: 1875
+---
+
+- Added `cursorless_insert` action to the public Talon api. This api enables you to define custom grammars for cursorless text insertion. See the [talon-side api docs](https://www.cursorless.org/docs/user/customization/#public-talon-actions) for more