Add form lib docs

[cjcenizal]

Direct conversion of content added via #78654 to a format compatible with our new docs system.

@sebelga I created this just to try it out. Pinging you in case you wanted to give the new docs system a whirl, so that we don't duplicate effort. Note that I had to massage the headings significantly, which was fairly tedious grepping.

This can be added to a locally running version of the docs site by adding it as a source:

// sources-dev.json

{
  "sources": [
    {
      "type": "github",
      "location": "elastic/design-manual"
    },
    {
      "type": "github",
      "location": "elastic/elastic-docs-playground"
    },
    {
      "type": "github",
      "location": "elastic/kibana-team"
    },
    {
      "type": "file",
      "location": "../kibana/src/plugins/es_ui_shared/docs"
    }
  ]
}

And then creating a new nav file and adding it to nav.ts:

// config/nav-form-lib.ts

import { DocsConfigNavShape } from './nav';

export const navFormLib: DocsConfigNavShape = {
  mission: 'Kibana development',
  id: 'kibanaDevelopment',
  slug: 'kibana/development',
  icon: 'logoKibana',
  description: 'Kibana developer tools and patterns',
  items: [
    {
      category: 'Plugin development',
      items: [{ id: 'kibanaFormLib' }],
    },
  ],
};

Here's what it looks like:

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 105b476

Metrics [docs]

distributable file count

id	before	after	diff
default	42758	42759	+1
oss	22452	22453	+1

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[sebelga]

Hey @cjcenizal , cool for the trial.
Let me know before putting too much effort into this as I have a current branch where I did the conversion to the doc system. I will continue once #78654 is merged.

For me, having one giant file is not a good solution, both for the reader and the DX, but I was going to suggest it to @snide when integrating. We should keep small, scoped files that are much easier to handle, reason about and review.