Introduce web_presentation annotation for documentation

[bartv]

The goal of this feature is to allow the model developer to provide information to the user/operator in the web console. The current proposal is to use the following in the model:

    string documentation
    lsm::attribute_modifier documentation__modifier="r"
    dict documentation__annotations = {
        "web_presentation": "documentation",
        "web_title": "Documentation",
        "web_icon": "fa-info-circle",
        "web_content_type": "text/markdown"
    }

The json equivalent

{
    "attribute_annotations": {
        "web_content_type": "text/markdown",
        "web_icon": "fa-info-circle",
        "web_presentation": "documentation",
        "web_title": "Documentation"
    },
    "default_value": null,
    "default_value_set": false,
    "description": null,
    "modifier": "r",
    "name": "documentation",
    "type": "string",
    "validation_parameters": null,
    "validation_type": null
}

This results in an additional tab in expanded row (before the others).

The support content types could/should be:

• text/plain
• text/html (do we want to do this??)
• text/markdown

For text/markdown we also want to support the mermaid extension so that we can include diagrams.

Discussion: do we use "tab" for the web_presentation annotation?

This also means we need to be able to render markdown in the browser. markdown-it (also used in the python world) seems to be one of the most complete and most used implementations:

For mermaid this is mermaid-js and there seems to be a number of plugins available. The original implementation of the plugin seems to be no longer maintained. However this fork seems to be more active: https://github.com/wekan/markdown-it-mermaid#readme

The following is some example code that you can use to test (without the backslashes):

# This is some more information

\```mermaid
  graph TD;
      A-->B;
      A-->C;
      B-->D;
      C-->D;
\```