docs(cli): add docs for custom plugins

[BioPhoton]

This PR adds documentation about custom plugins to the cli docs.

Notice:
I would like to get feedback in the following way:

1. the pr gets reviewed only for structure, sections and order

• reduce size of sections

2. the pr gets wording polish and rough feedback on code examples

• SEO
• persona centric wording
• reduce code complexity
• reduce code and types to a minimum
• add detail tags to reduce initially visual code
• code imports are added

3. the pr goes into ready for final review

• remove todos
• last check

I will also resolve the possible comments in the following order.

Todos:

• structure, sections and order is correct
  • First feedback
    introduced more sections on data strucutres and added links to other docs.
• Rework v2

• wording and code examples are polished
  • First feedback
    • added in-site links and notes to missing links
    • added intro section
    • add docs to file-size plugin example
  • added links and polished text

closes #117

[IKatsuba]

Left some comments

[matejchalk]

Overall I think this is a very good step-by-step guide. My only note on structure is I wouldn't include it the CLI README, because it's quite long and only relevant for plugin authors, making it more of an advanced topic. I would move into a separate Markdown file (e.g. docs/plugin-tutorial.md), and only include a link here.

Otherwise I had some more detailed notes in the comments.

[BioPhoton]

I had more thoughts on the chapters "Scoring of audits" and "Attribution of audits"... I think it is a better learning if we direclty stick to the data structure and I arite more in the context of audits.

So the new COT could look like this:

• 1. Custom Plugins
  • 1.1. Metadata
  • 1.1. Add a plugin to core config
  • 1.2. Runner
    • 1.2.1. RunnerFunction
    • 1.2.2. RunnerConfig
  • 1.3. AuditOutput
    • 1.3.1. Metadata
    • 1.3.1. Scoring
    • 1.3.2. Attribution
  • 1.4. Groups
• 2. Plugins and categories

[Tlacenka]

I like it!

Just wondering whether the removal of the second file is intentional, the text seemed well split where the CLI was a high-level introduction and the main custom plugins was more of a tutorial. 🤔

[Tlacenka]

Looking good!

A few TODOs are still left (links to models). Is this desired?

[Tlacenka]

Except for the todos, great improvement, well documented user journey towards custom plugins! 💯