docs: the repository mixes English and French documentation

[c0remusic]

The problem

The repository ships documentation in two languages:

• English: README.md, .github/CONTRIBUTING.md, .github/CODE_OF_CONDUCT.md,
  .github/SECURITY.md, issue and pull request templates;
• French: docs/architecture.md, docs/chord-types.md.

These two French files are the only technical documentation a contributor gets after the
README. Someone who reaches them from an English README hits a wall exactly where the
project starts being interesting.

Why it matters

The project is presented in English, has English topics, and takes issues in English. Two
French files in docs/ are not a stylistic detail — they are the reference material for
anyone who wants to touch the chord engine or the voice leading.

Definition of done

A decision, applied consistently:

• either translate docs/architecture.md and docs/chord-types.md to English, and
  keep everything English from now on;
• or state the project language explicitly in the README and accept the mix, with a
  note at the top of each French file.

Whichever is chosen, CONTRIBUTING.md should say what language new documentation is
expected in, so the question does not come back with every pull request.