[DOC] Update the CONVENTIONS.md with solid guidance of plugin naming convention

[ruanyl]

For the team that is new to the plugin development, the current documentation of repo name and package name naming convention is a bit unclear.

It would be nice to add clarifications on the naming conventions of:

1. GitHub repo name of OpenSearch/OpenSearch-Dashboards plugin
2. Package name of OpenSearch-Dashboards plugin (name in package.json)

[dblock]

👍 please help contribute a PR! you might find #174 relevant

[ruanyl]

Hi @dblock @joshuarrrr, drafting a PR for it #204

Based on the current documentation and my observations, the dashboard plugin repo name should have prefix dashboards, however, the plugin name and id generally have dashboards as suffix. Do you see this as misalignment due to historical reason or was it by intention?

[dblock]

Resolved via #204