Add documentation for the site-config.json keys

[kelvin-muchiri]

Closes #470

Documentation for the site-config.json keys

Add documentation for the keys:

• [x] APP
• [x] APP.oauthclientID
• [x] APP.mapConfig
• [x] APP.mapConfig.container
• [x] APP.mapConfig.style
• [x] APP.mapConfig.center
• [x] APP.mapConfig.zoom
• [x] APP.accessToken
• [x] APP.appIcon
• [x] APP.appName
• [x] APP.appColor
• [x] APP.appNameDesc
• [x] APP.appLoginIcon
• [x] APP.searchBar
• [x] APP.mapStateToUrl
• [x] APP.hasNavBar
• [ ] APP.supersetOnlyLogin
• [x] APP.mapConfig.mapBounds
• [x] APP.mapConfig.preserveDrawingBuffer
• [x] APP.apiAccessToken
• [x] APP.includeNavControls
• [x] APP.removeMapScale
• [x] APP.supersetBase
• [x] APP.authConfig
• [x] APP.authConfigApiMap
• [x] APP.showSinglePeriods
• [x] APP.layerSupersetLinks
• [x] APP.NULL_LAYER_TEXT
• [x] SUPERSET_CONFIGS
• [x] SUPERSET_CONFIGS.iframes
• [x] SUPERSET_CONFIGS.splashPage
• [x] STYLES
• [x] LAYERS
• [x] APP.hasNavBar

[kelvin-muchiri]

@kahummer I was thinking we should against each key if (Required) or (Optional) and those that are optional to appear after the required. e.g

accessToken (Required): Your Mapbox API access token.