[Bug]: Plugin documentation is unclear

[adamdicarlo0]

Current Behavior (bug)

The Creating plugins page does not say what the filename for a plugin must be. It just says "Here's what goes in your plugin file." I don't have the context to know what the filename should be. At first I thought maybe it should be devbox.json -- i.e. plugins were special devbox.jsons.

Looking in the devbox repository, the builtin plugins are all named different things. mysql.json, postgres.json, etc. Seeing that confused me more.

I eventually figured out that it is at least convention -- still not sure if it's required -- to name the file plugin.json.

• When using path: to test locally (please add instructions for this to the docs), you have to specify the json file itself, not just the directory. That seemed unintuitive--I figured I'd specify the directory and name the json file correctly and be on my way.
• When using github:, you don't specify the file name, just the repository and (optional?) ?dir= parameter. Wha? Why is it different? Is the name being plugin.json only required when specifying a plugin from github?

Do init_hooks actually work with plugins? (see #1500)

The Creating plugins page shows init_hook incorrectly located at the root of the JSON:

{
  "name": "",
  "version": "",
  "readme": "",
  "env": {
    "<key>": "<value>"
  },
  "create_files": {
    "<destination>": "<source>"
  },
  "init_hook": [              <--- must be nested in `shell` object
    "<bash commands>"
  ]
}

Expected Behavior (fix)

Creating plugins page describes how to:

• name the plugin's JSON file - are there requirements? Conventions? Can you name the file something different, and still "include" it from github?
• go about testing a plugin while creating it--mention the "path:" syntax, and whether devbox add can be used to install a plugin, or whether manual editing of devbox.json is necessary
• correctly identify init_hook's location in JSON -- shell.init_hook

shell.init_hook support was added in #1500 - thank you @mikeland73!

Creating plugins page has a link to jetpack-io/devbox-plugins as an example of plugins that are not the special-cased/hardcoded builtin variety.

Additional context
Devbox 0.5.13, running on a Linux system

Thank you for devbox! My team and I have been really liking it!

### Tasks
- [x] Clarify or recommend a plugin filename
- [ ] https://github.com/jetpack-io/devbox/pull/1500
- [x] Update testing information for local plugins

[adamdicarlo0]

I would say this is fixed! Thanks again!

[Lagoja]

Awesome, thank you for reporting!