Add initial docs for editor diagnostics (#33325)

Release Notes:

- N/A

docs/src/SUMMARY.md

@@ -34,6 +34,7 @@
 - [Collaboration](./collaboration.md)
 - [Git](./git.md)
 - [Debugger](./debugger.md)
+- [Diagnostics](./diagnostics.md)
 - [Tasks](./tasks.md)
 - [Remote Development](./remote-development.md)
 - [Environment Variables](./environment.md)

docs/src/diagnostics.md

@@ -0,0 +1,70 @@
+# Diagnostics
+
+Zed gets its diagnostics from the language servers and supports both push and pull variants of the LSP which makes it compatible with all existing language servers.
+
+# Regular diagnostics
+
+By default, Zed displays all diagnostics as underlined text in the editor and the scrollbar.
+
+Editor diagnostics could be filtered with the
+
+```json5
+"diagnostics_max_severity": null
+```
+
+editor setting (possible values: `"off"`, `"error"`, `"warning"`, `"info"`, `"hint"`, `null` (default, all diagnostics)).
+
+The scrollbar ones are configured with the
+
+```json5
+"scrollbar": {
+  "diagnostics": "all",
+}
+```
+
+configuration (possible values: `"none"`, `"error"`, `"warning"`, `"information"`, `"all"` (default))
+
+The diagnostics could be hovered to display a tooltip with full, rendered diagnostic message.
+Or, `editor::GoToDiagnostic` and `editor::GoToPreviousDiagnostic` could be used to navigate between diagnostics in the editor, showing a popover for the currently active diagnostic.
+
+# Inline diagnostics (Error lens)
+
+Zed supports showing diagnostic as lens to the right of the code.
+This is disabled by default, but can either be temporarily turned on (or off) using the editor menu, or permanently, using the
+
+```json5
+"diagnostics": {
+  "inline": {
+    "enabled": true,
+    "max_severity": null, // same values as the `diagnostics_max_severity` from the editor settings
+  }
+}
+```
+
+# Other UI places
+
+## Project Panel
+
+Project panel can have its entries coloured based on the severity of the diagnostics in the file.
+
+To configure, use
+
+```json5
+"project_panel": {
+  "diagnostics": "all",
+}
+```
+
+configuration (possible values: `"off"`, `"errors"`, `"all"` (default))
+
+## Editor tabs
+
+Similar to the project panel, editor tabs can be colorized with the
+
+```json5
+"tabs": {
+  "show_diagnostics": "off",
+}
+```
+
+configuration (possible values: `"off"` (default), `"errors"`, `"all"`)