chore: pglpgsql docs (#478)

Author: psteinroe

docs/plpgsql.md

@@ -0,0 +1,9 @@
+# PL/pgSQL Support
+
+Postgres Language Tools partially supports PL/pgSQL. We use `libpg_query` to parse the function body and show any syntax error. For a more sophisticated integration, make sure to enable the `plpgsql_check` extension in your development database.
+
+```sql
+CREATE EXTENSION IF NOT EXISTS plpgsql_check;
+```
+
+If the extension is detected, we leverage it to run more advanced checks against your PL/pgSQL functions.

mkdocs.yml

@@ -16,6 +16,7 @@ nav:
   - Introduction: index.md
   - Guides:
       - Linting Migrations: checking_migrations.md
+      - PL/pgSQL Support: plpgsql.md
       - Troubleshooting: troubleshooting.md
   - Reference:
       - Rules: rules.md