feat(data): move rebuildable local data with OPENIDEA_LOCAL_DATA_DIR (#2373)

- The local data directory (tools, language servers, index caches) is resolved in one place, ide_host::data_dirs::local_data_dir
- The variable must be an absolute path; a missing directory is created, and an unwritable one exits with an error instead of falling back to the system default
- Config and log directories still follow the system
- Add docs/data-directories.md

Co-authored-by: LU XIANGRUI <[email protected]>

docs/data-directories.md

@@ -0,0 +1,24 @@
+# Data directories
+
+OpenIDEA keeps its rebuildable data, such as downloaded tools, language servers,
+JDKs and index caches, in the operating system's local data directory:
+
+- **Windows:** `%LOCALAPPDATA%\ai.l0ng.openidea`
+- **macOS:** `~/Library/Application Support/ai.l0ng.openidea`
+- **Linux:** `~/.local/share/ai.l0ng.openidea`
+
+To keep this data elsewhere, for example on another drive, set
+`OPENIDEA_LOCAL_DATA_DIR` to an absolute path before starting OpenIDEA. The IDE
+then uses `<OPENIDEA_LOCAL_DATA_DIR>/ai.l0ng.openidea` and creates it when
+missing. If the path is relative or cannot be written, OpenIDEA shows an error
+and exits instead of falling back to the default location.
+
+```bat
+setx OPENIDEA_LOCAL_DATA_DIR "E:\OpenIDEA"
+```
+
+Settings, plugins, scratches and other user data stay in the operating system's
+data and configuration directories. Logs stay in their usual location, so they
+are still written when the variable is wrong. Existing data is not moved: quit
+OpenIDEA and move the old directory's contents to the new location, or let the
+IDE download tools and rebuild indexes again.