Documentation for semver upgrade edge constraints support

[m1kola]

Need to create user-facing documentation for #377 which is based on this RFC.

---

What we should include:

• Cover ForceSemverUpgradeConstraints feature gate and mention default state (enabled)
• Explain how semver upgrades work
  • Extensively cover behaviour of major version zero (see semver spec) which behaves differently fromv versions from >=1.0.0.
  • Explain that setting .spec.upgradeConstraintPolicy to Ignore is a way for upgrading versions in major version zero.
• Explain legacy OLMv0 semantics or link to the existing docs
  • Might be worth mentioning that at this time we only support Replaces and do not support Skips and SkipRange
• Explain what .spec.upgradeConstraintPolicy on Operator is for
  • Might be worth adding a warning that this need to be used carefully and independent verification for upgrades is required
  • Potentially mention ability to downgrade packages