Update PostgreSQL Maintenance page

Added a mid-sized VPS configuration with configuration changes to the PostgreSQL database config.

Deleted single quotes in one of the examples to unify the examples

docs/maintenance-postgres.md

@@ -108,11 +108,33 @@ These are not recommended values and they may not work well for you. This is jus
 Here is an example config for a small 2 core server with 4GB of RAM and SSD storage:
 ```
 matrix_postgres_process_extra_arguments: [
-  "-c 'shared_buffers=128MB'",
+  "-c shared_buffers=128MB",
-  "-c 'effective_cache_size=2304MB'",
+  "-c effective_cache_size=2304MB",
-  "-c 'effective_io_concurrency=100'",
+  "-c effective_io_concurrency=100",
-  "-c 'random_page_cost=2.0'",
+  "-c random_page_cost=2.0",
-  "-c 'min_wal_size=500MB'",
+  "-c min_wal_size=500MB",
+]
+```
+
+Here is an example config for a 4 core server with 8GB of RAM on a Virtual Private Server (VPS); the paramters have been configured using https://pgtune.leopard.in.ua with the following setup: PostgreSQL version 12, OS Type: Linux, DB Type: Mixed type of application, Data Storage: SSD storage:
+```
+matrix_postgres_process_extra_arguments: [
+  "-c max_connections=100",
+  "-c shared_buffers=2GB",
+  "-c effective_cache_size=6GB",
+  "-c maintenance_work_mem=512MB",
+  "-c checkpoint_completion_target=0.9",
+  "-c wal_buffers=16MB",
+  "-c default_statistics_target=100",
+  "-c random_page_cost=1.1",
+  "-c effective_io_concurrency=200",
+  "-c work_mem=5242kB",
+  "-c min_wal_size=1GB",
+  "-c max_wal_size=4GB",
+  "-c max_worker_processes=4",
+  "-c max_parallel_workers_per_gather=2",
+  "-c max_parallel_workers=4",
+  "-c max_parallel_maintenance_workers=2",
 ]
 ```