docs: add explanation for query parameters/replacements (#1381)

Author: ppcamp

docs/src/pages/en/features/executing-queries.mdx

@@ -27,3 +27,45 @@ You have 3 methods to execute queries. Run from selection, run from bookmarks, r
 1. Select `SQLTools History: Run` from command palette.
 2. Pick the query you want from the list.
 3. Wait for the results to show up.
+
+## Queries with parameters
+
+It also supports query string replacements with parameters either defined in `variables` or
+provided with `inputBoxes`.
+
+To enable query parameters replacements, you need to provide some additional configs.
+In in your `workspaceFolder`, edit the file `.vscode/settings.json`. Make sure that you have
+the following parameters enabled:
+
+```json
+"sqltools.queryParams.enableReplace": false, // enables string replacement.
+// the following regex, will match named params in your variables files
+"sqltools.queryParams.regex": "\\$[\\d]+|([@:$][\\w]+)",
+
+// a connection example...
+"sqltools.connections": [
+    {
+        "previewLimit": 50,
+        "server": "localhost",
+        "port": 5432,
+        "driver": "PostgreSQL",
+        "name": "MyDb",
+        "group": "Locals",
+        "database": "world-db",
+        "username": "world",
+        "password": "world123",
+        "askForPassword": false,
+        "variables": {
+            ":name": "'%e%'",
+        }
+    }
+]
+```
+
+In this case, when you execute your query, every word that matches `:name` will
+be automatically replaced for `'%e%'`. But if you have a param like `$1`, it'll
+ask you for it every time.
+
+**NOTE**: this is just a string replacement, and therefore, you can execute
+subqueries using this replacement. You can also change the query columns/filters
+itself.