Documentation: update the github wiki to include guidance for embedding images, images with alignment, and youtube videos

[jhung]

The IDRC website wiki should be updated to include how an author can add different media types. Specifically:

• add image
• image position with text
• YouTube embed
• File

Note: these features are only available in some and not all editors. For example they're available in Page and Project Body editors, and not Page Intro or Project Listing Description editors.

[jhung]

In the documentation, it may also be worth mentioning copying the YouTube embed code from the YouTube site will make the video not scale well on the website. Therefore embedding YouTube videos should be done with our helper.

Quote from @greatislander in response to a support question:

By default if you copy an embed code from YouTube, it includes a hard-coded width of (I believe) 560 pixels. That causes all of the content to overflow on small device screens and messes up the image sizing (which is styled so as to not exceed the container width… but if the device width is 320 pixels, for example, and the container is larger than that, it will appear too large).

We anticipated this issue with embedded video and there is a helper for adding embedded YouTube videos that will prevent them from overflowing on small sizes. When authoring content on the site, choose “YouTube Embed” from the toolbar menu...