Forms: Update or create documentation around form field structure and inner blocks

Part of: https://github.com/Automattic/jetpack/issues/42353
Project Thread: TBA

## Description

Switching Jetpack Form Fields to inner blocks will require comprehensive documentation updates so developers and end users can quickly understand the new architecture and customization options. This issue addresses the creation or revision of relevant docs, ensuring they reflect the final, inner block–based approach for Jetpack Forms.

## Scope & Requirements

- Update or create developer-facing documentation (e.g., READMEs, inline docs) to clarify how Jetpack Forms now use inner blocks.
- Revise any user-facing guides, knowledge-base articles, or release notes to describe the new form field structure, focusing on what changed and why.
- Provide examples or references for theme developers, agency partners, or advanced users who rely on custom form styling or shortcodes.

## Tasks

1. [ ] Review existing docs for outdated details
- Locate READMEs, inline block docs, or support articles referencing the old form block approach or structure.
- Make notes where references need to mention or illustrate inner blocks.
2. [ ] Create or update technical documentation
- Document each new block (Label, Input, Option, etc.) with usage instructions, attributes etc.
- (Optional) Include snippets demonstrating how forms can be customized or styled under the new architecture.
3. [ ] Revise user-facing guides (if applicable)
- If Jetpack has end-user guides on form creation or styling, update them to reference inner blocks.
- Highlight any noteworthy changes (e.g., global style overrides).

Start by reviewing existing Jetpack Forms documentation, READMEs, inline block docs, support articles, and release notes for references to the old form structure. Identify which materials need updates, then document the inner-block architecture, the Label/Input/Option blocks, customization guidance, and relevant user-facing changes; done means outdated references are revised and the new structure is explained with useful examples or references.

Written by the indexing model from the issue text.