Add documentation for the slack API

[daveoconnor]

When trying to run just manage release_tasks to import the data for release reports I got:

Updating slack activity buckets...
Traceback (most recent call last):
  File "/code/manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/venv/lib/python3.11/site-packages/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/venv/lib/python3.11/site-packages/django/core/management/__init__.py", line 436, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/venv/lib/python3.11/site-packages/djclick/adapter.py", line 68, in run_from_argv
    exit_code = self.main(
                ^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/djclick/adapter.py", line 50, in invoke
    return super(DjangoCommandMixin, self).invoke(ctx)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/code/libraries/management/commands/release_tasks.py", line 103, in command
    handled_commits = run_commands(progress)
                      ^^^^^^^^^^^^^^^^^^^^^^
  File "/code/slack/management/commands/fetch_slack_activity.py", line 234, in inner
    return fn(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^
  File "/code/libraries/management/commands/release_tasks.py", line 77, in run_commands
    call_command("fetch_slack_activity")
  File "/venv/lib/python3.11/site-packages/django/core/management/__init__.py", line 194, in call_command
    return command.execute(*args, **defaults)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/djclick/adapter.py", line 118, in execute
    self.invoke(ctx)
  File "/venv/lib/python3.11/site-packages/djclick/adapter.py", line 50, in invoke
    return super(DjangoCommandMixin, self).invoke(ctx)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/code/slack/management/commands/fetch_slack_activity.py", line 234, in inner
    return fn(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^
  File "/code/slack/management/commands/fetch_slack_activity.py", line 292, in command
    selected_channels.extend(get_my_channels())
  File "/code/slack/management/commands/fetch_slack_activity.py", line 32, in get_my_channels
    for page in client.conversations_list():
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/slack_sdk/web/client.py", line 3124, in conversations_list
    return self.api_call("conversations.list", http_verb="GET", params=kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/slack_sdk/web/base_client.py", line 157, in api_call
    return self._sync_send(api_url=api_url, req_args=req_args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/slack_sdk/web/base_client.py", line 188, in _sync_send
    return self._urllib_api_call(
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/slack_sdk/web/base_client.py", line 319, in _urllib_api_call
    ).validate()
      ^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/slack_sdk/web/slack_response.py", line 199, in validate
    raise e.SlackApiError(message=msg, response=self)
slack_sdk.errors.SlackApiError: The request to the Slack API failed. (url: https://slack.com/api/conversations.list)
The server responded with: {'ok': False, 'error': 'not_authed'}
error: Recipe `manage` failed on line 131 with exit code 1

Documentation should be added on how to set that up.