BB-628: Update onboarding course

This PR incorporates feedback from newcomers and improves the onboarding documentation.

Changelog:

• Added section on What is Ocim, and how to set up a Ocim Devstack
• Improved documentation on EdX, and explanation about releases
• Improved description of what devstack needs to be provisioned
• Created guides on how to set up each devstack: Docker, Vagrant and Solutions
• Fixed some broken links
• Added a small guide on how to log time and how to check for all time logged on Jira
• Changed secrets guide from KeePass to Hashicorp's Vault and included documentation
• Improved mailing list explanation

How to check:

1. Import this on your devstack
2. Check the changelog and see if the changes have been correctly done.

Reviewer:

• @pauloviadanna