doc: misc enhancements to contributors

[lukpueh]

Description of issue or feature request:
Here are a few suggestions to enhance our Instructions for Contributors on RTD.

Current behavior:

• No mention of Secure Systems Lab development guidelines
• Too extensive (and prominent) section about DCO
• Too extensive (and outdated/opinionated?) virtualenv description
• Very extensive description of test logging/verbosity config

Expected behavior:

• Link to SSLab development guidelines
• Shorten DCO section (and optionally move down in doc) (see e.g. in-toto Instructions for Contributors for inspiration)
• Just recommend usage of and link to existing virtualenv documentation
• Shorten description of test logging/verbosity config

[jku]

• Shorten description of test logging/verbosity config

#1724 is going to make tox run a reasonable approximation of CI tests so we should suggest that as the way to run tests.

Then after that we can include a short mention on how to run specific tests and the options (-v for logs, --dump for repository dumps when available)