Add markdown widget

[babbitt]

Apologies if anything is amiss here; first time working in go and first time contributing to this project. Happy to make style adjustments as needed.

Address #966 and creates the markdown widget which can either take content or a file to render.
Adds minimal CSS (I assume more will be requested but wanted to see if there was appetite for this change first.

Demo

Documentation snippet

(To help save time)

Markdown

Embed any Markdown content.

Properties

⚠️ Note that one of source or file is required.
file will take priority over source if both are given. An error will be thrown if neither are present.

Name	Type	Required	Default
source	string	no
file	string	no
title	string	no	"Markdown"

source

Inline Markdown content to be displayed.

file

Location of a Markdown file to be displayed.

title

Title of the widget; Markdown if left unset.

Example:

- type: markdown
  source: >
    # Hello

    Here is a some [markdown](https://www.markdownguide.org/) content