docs: Write DEVELOPING_PLUGINS.md (EN + PL)

[przemyslawbialon]

Labels: documentation, good first issue, community
Priority: P2
Blocked by: #4 (needs stable ABC), #5 (references cookiecutter)

Context

Once the plugin ABC is stable (#4) and the cookiecutter template exists (#5), a dedicated walkthrough document will close the loop for contributors who want to add a broker. The goal is: a developer with Python knowledge but no project context should be able to go from "I have XTB's export CSV" to "I have a merged PR" in under 4 hours.

Bilingual because the target audience is Polish developers, and "I speak English better than Polish for technical topics" is a valid preference both ways.

Acceptance Criteria

• docs/DEVELOPING_PLUGINS.md (English)
• docs/DEVELOPING_PLUGINS.pl.md (Polish, ideally mirrored structure)
• Covers these sections:
  1. What is a broker plugin? — one paragraph overview
  2. The BrokerPlugin contract — ABC reference with examples
  3. Step-by-step: Add a new broker (with cookiecutter)
     • Run cookiecutter gh:pbialon/pit-38-broker-template
     • Inspect the generated files
     • Get an example export from your broker
     • Write the CSV parser
     • Run tests
     • Submit a PR
  4. Common patterns — handling date parsing, currency conversion edge cases, missing columns
  5. Testing your plugin — how to use tests/utils.py helpers (StubExchanger, buy(), sell())
  6. Tax-correctness checklist — if your broker reports different numbers, which ones are canonical? (fee treatment, gross vs net amounts, dividend withholding)
  7. Getting help — link to Discussions (Add a port for revolut as a data source  #3)
• Includes at least 2 worked code examples (Revolut and Binance) showing the "before cookiecutter scaffolding → after full plugin" diff
• Linked from README.md, CONTRIBUTING.md (Create a service for business logic of stock transactions  #1), pit38/plugins/README.md

Technical Approach

Draft in English first, then translate. Use mermaid diagrams for the "flow of a transaction":

graph LR
    A[Broker CSV] --> B[RowParser]
    B --> C[Canonical CSV]
    C --> D[GenericCsvLoader]
    D --> E[Transaction objects]
    E --> F[Tax Calculator]

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Structure the "Common patterns" section around real issues hit when writing existing plugins — grep existing row_parser code for comments like # TODO, # HACK, # timezone handling, these are the learning opportunities to document.

Related

• Depends on Check which rates table from NBP we should use for exchange rates #4, Loss might be deductable from tax only for 5 years prior #5
• Replaces the thin pit38/plugins/README.md (which becomes a redirect)
• Establishes baseline for future docs/DEVELOPING_*.md (e.g. tax-rule docs)