docs(connectors): add section about adding built-in connectors (#1021)

* docs(connectors): add section about adding built-in connectors

* docs(connectors): update wording for conn. to be built in

Co-authored-by: Lovro Mažgon <lovro.mazgon@gmail.com>

* docs(connectors): update instructions for building in connectors

Co-authored-by: Lovro Mažgon <lovro.mazgon@gmail.com>

* docs(connectors): generalize built-in connector source

Co-authored-by: Lovro Mažgon <lovro.mazgon@gmail.com>

* docs(connector): use correct wording for dispenser factory

Co-authored-by: Lovro Mažgon <lovro.mazgon@gmail.com>

* docs(connectors): update code example to use diff highlighting

---------

Co-authored-by: Lovro Mažgon <lovro.mazgon@gmail.com>

docs/connectors.md

@@ -1,13 +1,18 @@
 # Conduit Connectors
 
-Connectors are an integral part of Conduit. Conduit ships with a couple of connectors that are built into the service
-to help developers bootstrap pipelines much more quickly. The built-in connectors include Postgres, File, Random Data
-Generator, Kafka and Amazon S3 connectors.
+Connectors are an integral part of Conduit. Conduit ships with a couple of
+[connectors that are directly built into](#built-in-connectors) the service,
+but it can also be expanded with additional
+[standalone connectors](#standalone-connectors).
 
-The difference between Conduit connectors and those you might find from other services is that Conduit connectors are
-Change Data Capture-first (CDC). CDC allows your pipeline to only get the changes that have happened over time instead
-of pulling down an entire upstream data store and then tracking diffs between some period of time. This is critical for
-building real-time event-driven pipelines and applications. But, we'll note where connectors don't have CDC capabilities.
+One of the main differences between Conduit connectors and those that you might find from other services is
+that all Conduit connectors are Change Data Capture-first (CDC).
+
+CDC allows your pipeline to only get the changes that have happened over time
+instead of pulling down an entire upstream data store and then tracking diffs
+between some period of time.
+This is critical for building real-time, event-driven pipelines and applications.
+But, we'll note where connectors do or do not have CDC capabilities.
 
 ## Roadmap & Feedback
 
@@ -18,12 +23,69 @@ Give the issue a `+1` if you really need that connector. The upvote will help th
 particular connector. If you find that an issue hasn't been created for your data store, please create a new issue in
 the Conduit repo.
 
-## Connectors
+## More about Connectors
+
+### Built-in connectors
+
+To help developers bootstrap pipelines much more quickly,
+Conduit ships with several built-in connectors by default.
+This includes the Postgres, File, Random Data Generator, Kafka and Amazon S3 connectors.
+
+If you are creating and distributing customized, pre-built Conduit binaries yourself,
+you may want to modify the list of built-in connectors, too.
+By modifying the list of built-in connectors,
+your compiled binary will include a set of pre-installed connectors specific to your end users' needs,
+and it can be run by others, as is, without them having to follow any additional installation instructions.
+
+If you want to create your own Conduit binary with a different set of built-in connectors,
+you can build-in an [existing connector](#the-list).
+Alternatively, you can also create your own
+[using the Conduit connector template or the Conduit Connector SDK](https://conduit.io/docs/connectors/building/).
+
+Once you have chosen a connector to be built-in, you can:
+
+- Download the new package and its dependencies: `go get "github.com/foo/conduit-connector-new"`
+- Import the Go module defining the connector
+into the [builtin registry](https://github.com/ConduitIO/conduit/blob/main/pkg/plugin/builtin/registry.go)
+and add a new key to `DefaultDispenserFactories`:
+
+```diff
+package builtin
+
+import (
+  // ...
+  file "github.com/conduitio/conduit-connector-file"
+  // ...
++ myNewConnector "github.com/foo/conduit-connector-new"
+)
+
+var (
+  // DefaultDispenserFactories contains default dispenser factories for
+  // built-in plugins. The key of the map is the import path of the module
+  // containing the connector implementation.
+  DefaultDispenserFactories = map[string]DispenserFactory{
+    "github.com/conduitio/conduit-connector-file":    sdkDispenserFactory(file.Connector),
+    // ...
++   "github.com/foo/conduit-connector-new":           sdkDispenserFactory(myNewConnector.Connector),
+  }
+)
+```
+
+- Run `make`
+- You now have a binary with your own, custom set of built-in connectors! 🎉
+
+### Standalone connectors
+
+In addition to built-in connectors, Conduit can be used together with standalone connectors
+to enable even more data streaming use cases.
+The Conduit team and other community developers create and maintain standalone connectors.
+
+Learn more about how you can install [standalone connectors to Conduit here](https://conduit.io/docs/connectors/installing).
 
 ### Support Types
 
 - `Conduit` - These are connectors that are built by the Conduit. Any issues or problems filed on those repos will be
-  respond to by the Conduit team.
+  responded to by the Conduit team.
 - `Community` - A community connector is one where a developer created a connector and they're the ones supporting it
   not the Conduit team.
 - `Legacy` - Some connectors are built using non-preferred methods. For example, Kafka Connect connectors can be used