jekyllrb.com site Troubleshooting updates

Addresses #3124. Adds an in-page nav to better surface available Troubleshooting content, and updates README with some useful information.

Per [comment](https://github.com/jekyll/jekyll/issues/3124#issuecomment-66125725) in #3124 I'd also like to beef up the Troubleshooting info for issues where a local Jekyll environment isn't working properly. Currently there is only one tip present in the docs, and there are other common cases that could be addressed, but I'll need some help with creating the content for that section.