[3.12] GH-107518: Remove the Argument Clinic How-To (#109900) (#110760)

(cherry picked from commit d1f7fae)

* Remove the content of the Argument Clinic HOWTO
* Update cross-references to the Argument Clinic
* Add a note directing readers to the devguide

Co-authored-by: Adam Turner <[email protected]>

Author: erlend-aasland