feat: Add docs for account-wide lambda-promtail subscriptions

[mchaffee-anaconda]

What this PR does / why we need it:

Currently, there are no examples for how to make lambda-promtail subscribe to a dynamic list of lambdas using log_group_names. It's common to need to ingest logs from "all current and future lambdas" rather than a static set.

This PR includes two examples for subscribing to a dynamic set of log groups.

The second example is a little rough due to weaknesses of this module such as the lack of outputs.tf, which we can fix in a later PR if deemed important enough of a use-case.

Special notes for your reviewer:

n/a

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR