Partial Set Security: Write documentation

[insumity]

Before PSS launches we need to:

• provide a feature overview
• provide validator documentation (joining, opting in etc)
• provide query and transaction documentation (REST, CLI)
• update the consumer chain onboarding documentation (https://cosmos.github.io/interchain-security/consumer-development/onboarding)
• add pseudocode on the math behind the power-cap shaping code

[p-offtermatt]

Some things to note:

• If all validators opt out of an opt-in chain, it will have a consensus failure and stop