Add docs for procedures

In the spirit of our planned move to concept-based documentation
rather than language-based documentation,
I've chosen to add a quick section to the overview,
and then a new page for documentation about procedures.
I have not updated any tutorials or reference pages.

Author: gefjon

docs/docs/01-Intro/01-overview.md

@@ -279,6 +279,115 @@ or at a specific time.
 </TabItem>
 </Tabs>
 
+### Procedure
+
+A **procedure** is a function exported by a [database](#database), similar to a [reducer](#reducer).
+Connected [clients](#client-side-sdks) can call procedures.
+Procedures can perform additional operations not possible in reducers, including making HTTP requests to external services.
+However, procedures don't automatically run in database transactions,
+and must manually open and commit a transaction in order to read from or modify the database state.
+
+<Tabs grouId="syntax" queryString>
+<TabItem value="rust" label="Rust">
+
+A procedure can be defined in a Rust module:
+
+```rust
+#[spacetimedb::procedure]
+pub fn make_request(ctx: &mut spacetimedb::ProcedureContext) -> String {
+    // ...
+}
+```
+
+And a Rust [client](#client) can call that procedure:
+
+```rust
+fn main() {
+    // ...setup code, then...
+    ctx.procedures.make_request();
+}
+```
+
+A Rust [client](#client) can also register a callback to run when a procedure call finishes, which will be invoked with that procedure's return value:
+
+```rust
+fn main() {
+    // ...setup code, then...
+    ctx.procedures.make_request_then(|ctx, res| {
+        match res {
+            Ok(string) => log::info!("Procedure `make_request` returned {string}"),
+            Err(e) => log::error!("Procedure  `make_request` failed! {e:?}"),
+        }
+    })
+}
+```
+
+</TabItem>
+<TabItem value="csharp" label="C#">
+
+C# modules currently cannot define procedures. Support for defining procedures in C# modules will be released shortly.
+
+A C# [client](#client) can call a procedure defined by a Rust or TypeScript module:
+
+```csharp
+void Main()
+{
+    // ...setup code, then...
+    ctx.Procedures.MakeRequest();
+}
+```
+
+A C# [client](#client) can also register a callback to run when a procedure call finishes, which will be invoked with that procedure's return value:
+
+```csharp
+void Main()
+{
+    // ...setup code, then...
+    ctx.Procedures.MakeRequestThen((ctx, res) =>
+    {
+        if (res.IsSuccess)
+        {
+            Log.Debug("Procedure `make_request` returned {Value}", res.Value);
+        }
+        else
+        {
+            throw new Exception(res.Error, "Procedure `make_request` failed!");
+        }
+    });
+}
+```
+
+</TabItem>
+<TabItem value="typescript" label="TypeScript">
+
+A procedure can be defined in a TypeScript module:
+
+```typescript
+spacetimedb.procedure("make_request", t.string(), ctx => {
+   // ...
+})
+```
+
+And a TypeScript [client](#client) can call that procedure:
+
+```typescript
+ctx.procedures.makeRequest();
+```
+
+A Rust [client](#client) can also register a callback to run when a procedure call finishes, which will be invoked with that procedure's return value:
+
+```typescript
+ctx.procedures.makeRequest().then(
+    res => console.log(`Procedure make_request returned ${res}`),
+    err => console.error(`Procedure make_request failed! ${err}`),
+);
+```
+
+</TabItem>
+</Tabs>
+
+See [Procedures](/procedures) for more details about procedures.
+
 ### Client
 
 A **client** is an application that connects to a [database](#database). A client logs in using an [identity](#identity) and receives an [connection id](#connectionid) to identify the connection. After that, it can call [reducers](#reducer) and query public [tables](#table).