Add documentation for plugins

[reenberg]

Added a markdown file in the docs folder that tries to explain how the plugin settings work.

Added categories for the settings and ordered them such that related properties are listed close to each other.

Closes: #326

[reenberg]

Hmm, I see that it ended up making the changes to packages.json quite messy to review.. let me see if I can make that better somehow.

[reenberg]

Hmm, I am unsure how to make the diff easier to review, without having to break it up into multiple commits, which could then be squashed later, when review is approved.

But i tried to pipe the package.json file through jq, to visualise how the properties is grouped, and to ensure nothing got lost. There are currently 19, and this change still has 19.

$ jq -r '.contributes.configuration[] | {title: .title,  order: .order, properties: .properties | keys_unsorted}  ' package.json 
{
  "title": "General",
  "order": 10,
  "properties": [
    "hediet.vscode-drawio.offline",
    "hediet.vscode-drawio.online-url",
    "hediet.vscode-drawio.codeLinkActivated",
    "hediet.vscode-drawio.local-storage",
    "hediet.vscode-drawio.simpleLabels",
    "hediet.vscode-drawio.zoomFactor",
    "hediet.vscode-drawio.globalVars",
    "hediet.vscode-drawio.resizeImages"
  ]
}
{
  "title": "Plugins",
  "order": 20,
  "properties": [
    "hediet.vscode-drawio.plugins",
    "hediet.vscode-drawio.knownPlugins",
    "hediet.vscode-drawio.customLibraries"
  ]
}
{
  "title": "Theme and styles",
  "order": 30,
  "properties": [
    "hediet.vscode-drawio.styles",
    "hediet.vscode-drawio.defaultVertexStyle",
    "hediet.vscode-drawio.defaultEdgeStyle",
    "hediet.vscode-drawio.colorNames",
    "hediet.vscode-drawio.presetColors",
    "hediet.vscode-drawio.customColorSchemes",
    "hediet.vscode-drawio.customFonts",
    "hediet.vscode-drawio.theme"
  ]
}