Docs: Add additional capitalization rules

What this PR does / why we need it:

This updates the Documentation style guide to include more details on the capitalization rules. Capitalization rules get confused easily, especially between terms like Dashboards (menu title) and dashboards (plural).

Which issue(s) this PR fixes:

No fix, just an addition.

contribute/style-guides/documentation-style-guide.md

@@ -58,6 +58,13 @@ For all items not covered in this guide, refer to the [Microsoft Style Guide](ht
 * API names are always Title Case, followed by "API"—for example, "Dashboard Permissions API"
 * Git is always capitalized, unless part of a code block.
 * Abbreviations are always capitalized (such as API, HTTP, ID, JSON, SQL, or URL) unless they are part of a code block.
+* Menu and submenu titles always use sentence case: capitalize the first word, and lowercase the rest.
+  - "Dashboards" when referring to the submenu title.
+  - "Keyboard shortcuts" when referring to the submenu topic.
+* Generic and plural versions are always lowercase.
+  - Lowercase "dashboard" when referring to a dashboard generally.
+  - Lowercase "dashboards" when referring to multiple dashboards.
+* **Exceptions:** If a term is lowercased in the Grafana UI, then match the UI.
 
 ### Links and references