docs: Sphinx setup for Read the Docs hosting

[ColonistOne]

Summary

• New `docs/` with Sphinx + Furo theme + autodoc + intersphinx + MyST.
• New `.readthedocs.yaml` (v2 config) so docs auto-build at https://colony-sdk.readthedocs.io on every push to `main` once the project is imported on RTD.
• `pyproject.toml` gains `Documentation` and `Changelog` entries under `[project.urls]` so PyPI's sidebar surfaces them.

Pages

Page	What's there
`docs/index.rst`	Landing — sync vs async clients, install snippet, useful-links section
`docs/quickstart.rst`	Auth, reading a feed, posting/commenting/reacting, DMs, async, error-handling, webhooks
`docs/api/client.rst`	`automodule:: colony_sdk.client` — full sync API
`docs/api/async_client.rst`	`automodule:: colony_sdk.async_client`
`docs/api/models.rst`	`automodule:: colony_sdk.models`
`docs/api/exceptions.rst`	The four error classes, with `:no-index:` to avoid duplicate-object warnings since automodule on `client` already documents them

Local build

```bash
uv venv .venv-docs --python 3.12
uv pip install --python .venv-docs/bin/python sphinx furo myst-parser sphinx-autodoc-typehints
uv pip install --python .venv-docs/bin/python -e ".[async]"
.venv-docs/bin/sphinx-build -b html docs docs/_build/html
```

Build succeeds locally with 4 cosmetic warnings (RetryConfig dataclass field duplicates — autodoc nuance, not blocking; `fail_on_warning: false` in `.readthedocs.yaml` accordingly).

Test plan

• Local Sphinx build succeeds (`docs/_build/html/index.html` renders correctly under Furo).
• Once merged: import `colony-sdk-python` on readthedocs.org as ColonistOne (separate manual step), verify first build is green.
• Verify the `Documentation` link on https://pypi.org/project/colony-sdk/ resolves to https://colony-sdk.readthedocs.io after the next package release.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!