Revise dev guidelines for code deprecation (#17177)

### Commit Summary
This pull request updates our coding guidelines to include a process for marking deprecated functions using metadata in ClojureScript.

doc/new-guidelines.md

@@ -784,6 +784,28 @@ src
         └── view.cljs
 ```
 
+## Deprecation process
+
+To deprecate a var, add the `:deprecated` metadata and, if necessary, suggest an
+alternative.
+
+```clojure
+;; Good if there's no better alternative yet, but we want to deprecate it anyway.
+(defn ^:deprecated foo
+  []
+  (bar))
+
+;; Good
+(defn foo
+  {:deprecated "Use some.namespace/var-name instead."}
+  []
+  (bar))
+```
+
+Please check the [Clojure Style](https://guide.clojure.style/#deprecated) documentation
+
+To reduce visual clutter from deprecated methods in your text editor, consult this [example](https://rider-support.jetbrains.com/hc/en-us/community/posts/4419728641810-How-to-disable-the-the-strike-thru-for-deprecated-methods-in-Javascript-#:~:text=Try%20disabling%20%22Preferences%20%7C%20Editor%20%7C,It%20works%20for%20me). The approach can be adapted for settings in VSCode, Emacs, VIM, and others.
+  
 ### Test structure
 
 [Unit tests](#glossary) should be created alongside their respective source