select docs refactoring, added $count docs and query utils section

Author: AlexBlokh

src/components/markdown/$count.mdx

@@ -0,0 +1,34 @@
+import Section from '@components/markdown/Section.astro';
+
+`db.$count()` is a utility wrapper of `count(*)`, it is a very flexible operator which can be used as is or as a subquery, more details in our [GitHub discussion](https://github.com/drizzle-team/drizzle-orm/discussions/3119).
+
+<Section>
+```ts
+const count = await db.$count(users);
+//    ^? number
+
+const count = await db.$count(users, eq(users.name, "Dan")); // works with filters
+```
+```sql
+select count(*) from "users";
+select count(*) from "users" where "name" = 'Dan';
+```
+</Section>
+
+It is exceptionally useful in [subqueries](/docs/select#select-from-subquery):
+
+```ts
+const users = await db.select({
+  ...users,
+  postsCount: db.$count(posts, eq(posts.authorId, users.id)),
+}).from(users);
+```
+
+usage example with [relational queries](/docs/rqb)
+```ts 
+const users = await db.query.users.findMany({
+  extras: {
+    postsCount: db.$count(posts, eq(posts.authorId, users.id)),
+  },
+});
+```

src/content/documentation/docs/_meta.json

@@ -67,6 +67,7 @@
   ["update", "Update"],
   ["delete", "Delete"],
   ["operators", "Filters"],
+  ["query-utils", "Utils"],
   ["joins", "Joins"],
   ["sql", "Magic sql`` operator"],
   "Performance",

src/content/documentation/docs/query-utils.mdx

@@ -0,0 +1,12 @@
+import Tab from '@components/markdown/Tab.astro';
+import Tabs from '@components/markdown/Tabs.astro';
+import Callout from '@components/markdown/Callout.astro';
+import Section from '@components/markdown/Section.astro';
+import IsSupportedChipGroup from '@mdx/IsSupportedChipGroup.astro';
+import $count from '@mdx/$count.mdx';
+
+# Drizzle query utils
+
+### $count
+<$count/>
+