Add support for important notes to the changelog generator

[tjerman]

The current version knows all of the supported categories.
What I forgot to support were important upgrade notes.

Inside of the JSON (example -- https://github.com/cortezaproject/corteza-docs/blob/2022.9.x/src/modules/ROOT/pages/changelog/202209/02.json) we need to add a new thing upgradeNotes (perhaps inside the meta or along side it) with the same content rules as other categories.

Refs

Plugin https://github.com/cortezaproject/corteza-docs/tree/2022.9.x/lib/changelogenator
Example https://github.com/cortezaproject/corteza-docs/blob/2022.9.x/src/modules/ROOT/pages/changelog/202209/02.gen.adoc#2022_9_2-important

[tjerman]

Here is an example of using it:

"important": [
    {
      "note": [
        "There is a known bug where multi-value fields crash the server or produce unexpected results in reports.",
        "If your configuration uses reports which utilize multi-value fields, skip this patch version."
      ]
    }
  ],