[Stack Monitoring] Add readme and dev_docs to monitoring plugin

[neptunian]

Similar to other plugins like APM or Fleet we should have documentation about how the app works. We can add topics we think should exist to this list.

• General README. Description of plugin, testing, explanation of collection types (legacy, metricbeat, agent), migration, etc
• a dev_docs directory with documentation on more complex topics that need more explaining
  • Rules / Alerting [Stack Monitoring] add dev doc about rules and alerts #123840
  • CCS
  • ES queries

Feel free to edit this description and add to the list of topics

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)

[matschaffer]

Guessing some of https://github.com/elastic/observability-dev/tree/main/docs/monitoring (internal) can probably move into the kibana repo as well as part of this effort.

[smith]

Fixed in #127420.