Revisit SDM operator docs

This is a follow-up on #21211

The following points need to be addressed

1. Revisit the support for Environment Variables (EVs).
- Link https://github.com/ethereum-optimism/optimism/pull/21211#discussion_r3465997899
- Pending on https://github.com/ethereum-optimism/optimism/issues/21360
- [ ] Make sure all options are supported via EVs
- [ ] Make sure all EVs are documented.
2. Update release-related documentation.
- Link https://github.com/ethereum-optimism/optimism/pull/21211#discussion_r3465971105
- Pending on Publishing a page on the activating u20 (likely).
- [ ] Create a reference to the u20 page in the docs and check all places mentioning e.g. update, u20, lagoon, activation, version.