Consolidating docs

[tcn33]

This removes documentation that has been consolidated into the Ruby Australia wiki:

• docs/blog-post-guidelines.md → https://github.com/rubyaustralia/committee/wiki/Blogging
• docs/events_and_campaigns.md → https://github.com/rubyaustralia/committee/wiki/Events-And-Campaigns
• docs/onboarding_new_committee_members.md → https://github.com/rubyaustralia/committee/wiki/CommitteeOnboarding#tldr and https://github.com/rubyaustralia/committee/wiki/Adding-Yourself-to-the-Website