Update contact page

- Update mailing list info
- Refine IRC section and add some extra tips
Add label for mailing list section
Fix freeseer/freeseer#266 layout.html

- Fix link to contact page.
  Note that it won't work locally anymore, but it will work on the live site.
- Simplify link to open an issue.
Update documentation.html page

- Simplify and reduce some of the text
- Add links where appropriate
- Fix some incorrect links