Write documentation about Pre-generate validator registrations

[boulder225]

🎯 Problem to be solved

We need to write docs explaining our new feature, ie, pre-generated validator registrations. But since this feature is compatible only with config v1.7 which will be released in release v0.17.0, we need to add corresponding docs before releasing the v0.17 docs version.

🛠️ Proposed solution

• [ ] Write documentation about https://github.com/ObolNetwork/charon/issues/2172 in our obol-docs