[SEP-033][P1] Write 'Add Your Own Transform' contributor guide

[samudzi]

Summary

Create a step-by-step guide showing how to write a custom ODS-E transform for a new data source (inverter brand, SCADA historian, API format). This is the key contributor funnel — turning users into contributors.

Motivation

MCP's growth was partly driven by community-contributed servers. ODS-E needs the same contributor flywheel for transforms. Currently, adding a new transform requires reading existing transformer source code and reverse-engineering the pattern. A clear guide lowers the barrier to contribution.

Proposed Content

1. When to write a transform — Your data source isn't covered by the generic CSV mapper
2. Transform anatomy — BaseTransformer class, required methods, record output format
3. Step-by-step:
   • Copy the template transform
   • Implement parse_row() and get_metadata()
   • Register the source alias
   • Add test fixtures
   • Run the test suite
4. Testing your transform — Required test cases, fixture format, CI expectations
5. Submitting your transform — PR checklist, SEP process for new transforms

Acceptance Criteria

• Published at /docs/guides/add-transform/
• Includes a downloadable transform template
• A developer unfamiliar with the codebase can follow it to add a new transform
• Covers testing requirements and PR checklist

Dependencies

• SEP-020 (generic CSV transform — explains when custom > generic)
• SEP-032 (architecture page — referenced for context)