Move screenshots directory to docs directory

[rolandwalker]

Description

Motivation: tidying the repository root.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

No blocking findings in this PR.

• I did not find correctness regressions or security issues in the changed lines.
• The path migration from screenshots/ to doc/screenshots/ appears consistently updated across README, MANIFEST.in, and affected tests.

Residual risk / testing gap:

• I couldn’t execute the touched tests locally because pytest is not installed in this environment (pytest: command not found).
• Suggest confirming CI covers:
  1. test_source_eager_completion
  2. test_source_leading_dot_suggestions_completion
  3. test_cd_command_with_one_real_folder_name