Fix link + better description for adding blog posts social sharing image
- Follow up from past releases - PMs always ask me where should they take the social sharing image from
- Last thread here: https://gitlab.slack.com/archives/C3TRESYPJ/p1536758734000100
- Fixes the RP handbook, linking to the correct part of the blog handbook that describes this process
- Link to a quick video tutorial I recorded for having a visual reference. Video is meh, and no biggie, but I guess it's better than having to figure it out by themselves
@rebecca are you ok with the changes to the blog handbook?