chore(external docs): improve vrl.dev tutorial (#21619)

* chore(external docs): improve vrl.dev tutorial

* indent yaml

* fix condition

* fix indentation

website/content/en/docs/reference/vrl/_index.md

@@ -25,14 +25,15 @@ VRL programs act on a single observability [event](#event) and can be used to:
 Those programs are specified as part of your Vector [configuration]. Here's an
 example `remap` transform that contains a VRL program in the `source` field:
 
-```toml {title="vector.toml"}
-[transforms.modify]
-type = "remap"
-inputs = ["logs"]
-source = '''
-  del(.user_info)
-  .timestamp = now()
-'''
+```YAML {title="vector.yaml"}
+transforms:
+  modify:
+    type: remap
+    inputs:
+      - logs
+    source: |
+      del(.user_info)
+      .timestamp = now()
 ```
 
 This program changes the contents of each event that passes through this
@@ -50,30 +51,49 @@ HTTP log events that look like this:
 }
 ```
 
-You want to apply these changes to each event:
+Let's assume you want to apply a set of changes to each event that arrives to your Remap transform in order to produce
+an event with the following fields:
 
-- Parse the raw string into JSON
-- Attempt to convert the timestamp and checks if the conversion was successful
-- If the conversion is successful, convert the time to a UNIX timestamp; otherwise, use the current time
-- Remove the `username` field
-- Remove the temporary timestamp (`parsed_timestamp`) field
-- Convert the `message` to lowercase
+- `message` (string)
+- `status` (int)
+- `timestamp` (int)
+- `timestamp_str` (timestamp)
 
-This VRL program would accomplish all of that:
+The following VRL program demonstrates how to achieve the above:
 
 ```coffee
+# Parse the raw string into a JSON object, this way we can manipulate fields.
 . = parse_json!(string!(.message))
 
-.parsed_timestamp = parse_timestamp!(.timestamp, format: "%Y-%m-%dT%H:%M:%S.%fZ")
-
-if is_timestamp(.parsed_timestamp) {
-  .timestamp = to_unix_timestamp(.parsed_timestamp)
+# At this point `.` is the following:
+#{
+#  "message": "SUCCESS",
+#  "status": 200,
+#  "timestamp": "2021-03-01T19:19:24.646170Z",
+#  "username": "ub40fan4life"
+#}
+
+# Attempt to parse the timestamp that was in the original message.
+# Note that `.timestamp` can be `null` if it wasn't present.
+parsed_timestamp, err = parse_timestamp(.timestamp, format: "%Y-%m-%dT%H:%M:%S.%fZ")
+
+# Check if the conversion was successful. Note here that all errors must be handled, more on that later.
+if err == null {
+   # Note that the `to_unix_timestamp` expects a `timestamp` argument.
+   # The following will compile because `parse_timestamp` returns a `timestamp`.
+  .timestamp = to_unix_timestamp(parsed_timestamp)
 } else {
+  # Conversion failed, in this case use the current time.
   .timestamp = to_unix_timestamp(now())
 }
 
+# Convert back to timestamp for this tutorial.
+.timestamp_str = from_unix_timestamp!(.timestamp)
+
+# Remove the `username` field from the final target.
 del(.username)
-del(.parsed_timestamp)
+
+# Convert the `message` to lowercase.
 .message = downcase(string!(.message))
 ```
 
@@ -83,7 +103,8 @@ Finally, the resulting event:
 {
   "message": "success",
   "status": 200,
-  "timestamp": 1614626364
+  "timestamp": 1614644364,
+  "timestamp_str": "2021-03-02T00:19:24Z"
 }
 ```
 
@@ -94,19 +115,21 @@ event. But you can also use VRL to specify conditions, which convert events into
 a single Boolean expression. Here's an example [`filter`][filter] transform that
 filters out all messages for which the `severity` field equals `"info"`:
 
-```toml {title="vector.toml"}
-[transforms.filter_out_info]
-type = "filter"
-inputs = ["logs"]
-condition = '.severity != "info"'
+```yaml {title="vector.yaml"}
+transforms:
+  filter_out_info:
+    type: filter
+    inputs:
+      - logs
+    condition: '.severity != "info"'
 ```
 
 Conditions can also be more multifaceted. This condition would filter out all
 events for which the `severity` field is `"info"`, the `status_code` field is
 greater than or equal to 400, and the `host` field isn't set:
 
 ```coffee
-condition = '.severity != "info" && .status_code < 400 && exists(.host)
+condition = '.severity != "info" && .status_code < 400 && exists(.host)'
 ```
 
 {{< info title="More VRL examples" >}} You can find more VRL examples further