adding contribute page on website.

[h4l0gen]

This will add contribute page with following content:

• Contributor workflow
• Your First Contribution (to fulfill CNCF assessment criteria)
• DCO
• TUF Repositories
• Slack channel
• Mailing list
• Community meeting
• Gvernance

It seems, I messed up config.toml file for reviewing 🥲. Let me clarify what I have done in config.toml file.

• I've added contribute page before Reporting issue page
• changed weights of order.
• I've removed previous Contribute page as it only redirects to python-tuf repo, and we want detailed contributing page.

Thanks for understanding and reviewing. 🙏

fixes #49