Docs: document per-project language_backend override; move field to SharedConfig

- Add "Per-Project Configuration" section to JetBrains plugin docs
  with cross-reference anchor for linking from other pages
- Add language_backend to project configuration feature list in workflow docs
- Add cross-reference from global config docs to per-project override
- Move language_backend field from ProjectConfig to SharedConfig,
  following the same pattern as symbol_info_budget (review feedback)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/02-usage/040_workflow.md

@@ -46,12 +46,13 @@ After creation, you can adjust the project settings in the generated `.serena/pr
 within the project directory.
 
 The file allows you to configure ...
-  * the set of programming languages for which language servers are spawned (not relevant when using the JetBrains plugin)  
+  * the set of programming languages for which language servers are spawned (not relevant when using the JetBrains plugin)
     Note that you can dynamically add/remove language servers while Serena is running via the [Dashboard](060_dashboard).
+  * the [language backend](per-project-language-backend) to use for this project (overriding the global setting)
   * the encoding used in source files
   * ignore rules
   * write access
-  * an initial prompt that shall be passed to the LLM whenever the project is activated 
+  * an initial prompt that shall be passed to the LLM whenever the project is activated
   * the name by which you want to refer to the project (relevant when telling the LLM to dynamically activate the project)
   * the set of tools and modes to use by default