Fixes to the site README.

[benlk]

Changes proposed:

• adds missing code fence, fixing problem where the code/not-code formatting was reversed for much of the file.
• fixes URL for the officers page
• fixes URLs for issues and pull requests to point to the OSUOSC repo instead of the theme's creator's repo.
• removes instructions from the readme that were specific to issues being filed against the theme's creator's repo
• groups new-post and new-page instructions under a new header, "New Content"

For a live version of this README, compare https://github.com/OSUOSC/website/tree/benlk-readme-cleanup with https://github.com/OSUOSC/website

@OSUOSC/website-team