docs: Document billing command (#4915)

* docs: Document billing command

* docs: Edits

* docs: Clarify inclusive/exclusive date format

* Apply batch of suggestions from code review

Co-authored-by: Robin Beck <stellarsquall@users.noreply.github.com>

* docs: Minor edit

---------

Co-authored-by: Robin Beck <stellarsquall@users.noreply.github.com>

website/content/docs/commands/billing/index.mdx

@@ -0,0 +1,45 @@
+---
+layout: docs
+page_title: billing - Command
+description: |-
+  The "billing" command retrieves the number of active users to help you predict user-based pricing.
+---
+
+# billing
+
+Command: `boundary billing`
+
+The `billing` command retrieves the number of active Boundary users in your organization to help you predict billing information.
+
+An active user is any unique Boundary user resource that has performed at least one authenticated action during the relevant billing period.
+Users perform authenticated actions when they call API endpoints that require a valid auth token.
+Boundary provides auth tokens to users when they authenticate using an auth method.
+
+## Examples
+
+The following command retrieves the number of monthly active users between September 2023 and February 2024:
+
+```shell-session
+$ boundary billing monthly-active-users /
+     -start-time="2023-09" /
+     -end-time="2024-02"
+```
+
+## Usage
+
+<CodeBlockConfig hideClipboard>
+
+```shell-session
+Usage: boundary billing [sub command] [options] [args]
+
+  # ...
+
+Subcommand:
+    monthly-active-users       Display the number of monthly active users
+```
+
+</CodeBlockConfig>
+
+For more information, examples, and usage, click on the name of the subcommand in the sidebar or the link below:
+
+- [monthly-active-users](/boundary/docs/commands/billing/monthly-active-users)

website/content/docs/commands/billing/monthly-active-users.mdx

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+---
+layout: docs
+page_title: billing monthly-active-users - Command
+description: |-
+  The "billing monthly-active-users" command retrieves the number of monthly active users during a specified time to help you predict user-based pricing.
+---
+
+# billing monthly-active-users
+
+Command: `boundary billing monthly-active-users`
+
+The `billing monthly-active-users` command retrieves the number of monthly active Boundary users in your organization during a specific time to help predict billing information.
+
+## Example
+
+The following command retrieves the number of monthly active users between September 2023 and February 2024:
+
+```shell-session
+$ boundary billing monthly-active-users /
+     -start-time="2023-09" /
+     -end-time="2024-02"
+```
+
+## Usage
+
+<CodeBlockConfig hideClipboard>
+
+```shell-session
+$ boundary billing monthly-active-users [options] [args]
+```
+
+</CodeBlockConfig>
+
+### Command options
+
+- `end-time=<string>` -  The end date for which you want to view monthly active users in YYYY-MM format.
+If you do not include an end time, the command retrieves active users from the start time through the current month.
+
+   The end time is exclusive, so the command returns monthly active users up to, but not including, the month you enter.
+   If the end time is **2024-07**, for example, the results include all active users from June (2024-06), but they would not include any users from July (2024-07).
+- `start-time=<string>` -  The start date for which you want to view monthly active users in YYYY-MM format.
+
+   The start time is inclusive, so any active users from the month you enter are included in the results.
+   If the start time is **2024-06**, for example, the command returns all users from June (2024-06) through the specified end time.
+
+@include 'cmd-option-note.mdx'

website/data/docs-nav-data.json

@@ -819,6 +819,19 @@
           }
         ]
       },
+      {
+        "title": "billing",
+        "routes": [
+          {
+            "title": "Overview",
+            "path": "commands/billing"
+          },
+          {
+            "title": "monthly-active-users",
+            "path": "commands/billing/monthly-active-users"
+          }
+        ]
+      },
       {
         "title": "authenticate",
         "routes": [