docs: Add API reference docs

[duwenxin99]

1. Set up API reference documents
2. Update github workflow to test for documentation correctness with nox
3. Add change log, html template, and convert README file from .md to .rst
4. A bunch of other small changes

[averikitsch]

Looks like when loading the library it's looking for our Google credentials. Github actions does not have our Google credentials. We need to fix the initialization from client: spanner.Client = spanner.Client() to client: Optional[spanner.Client] = None,:

File "/home/runner/work/langchain-google-spanner-python/langchain-google-spanner-python/src/langchain_google_spanner/__init__.py", line 15, in <module>
    from langchain_google_spanner.chat_message_history import SpannerChatMessageHistory
  File "/home/runner/work/langchain-google-spanner-python/langchain-google-spanner-python/src/langchain_google_spanner/chat_message_history.py", line 50, in <module>
    class SpannerChatMessageHistory(BaseChatMessageHistory):
  File "/home/runner/work/langchain-google-spanner-python/langchain-google-spanner-python/src/langchain_google_spanner/chat_message_history.py", line 136, in SpannerChatMessageHistory
    client: spanner.Client = spanner.Client(),
  File "/home/runner/work/langchain-google-spanner-python/langchain-google-spanner-python/.nox/docs/lib/python3.10/site-packages/google/cloud/spanner_v1/client.py", line 167, in __init__
    super(Client, self).__init__(
  File "/home/runner/work/langchain-google-spanner-python/langchain-google-spanner-python/.nox/docs/lib/python3.10/site-packages/google/cloud/client/__init__.py", line 320, in __init__
    _ClientProjectMixin.__init__(self, project=project, credentials=credentials)
  File "/home/runner/work/langchain-google-spanner-python/langchain-google-spanner-python/.nox/docs/lib/python3.10/site-packages/google/cloud/client/__init__.py", line 268, in __init__
    project = self._determine_default(project)
  File "/home/runner/work/langchain-google-spanner-python/langchain-google-spanner-python/.nox/docs/lib/python3.10/site-packages/google/cloud/client/__init__.py", line 287, in _determine_default
    return _determine_default_project(project)
  File "/home/runner/work/langchain-google-spanner-python/langchain-google-spanner-python/.nox/docs/lib/python3.10/site-packages/google/cloud/_helpers/__init__.py", line 152, in _determine_default_project
    _, project = google.auth.default()
  File "/home/runner/work/langchain-google-spanner-python/langchain-google-spanner-python/.nox/docs/lib/python3.10/site-packages/google/auth/_default.py", line 6[91](https://github.com/googleapis/langchain-google-spanner-python/actions/runs/8913558439/job/24479281774?pr=59#step:5:92), in default
    raise exceptions.DefaultCredentialsError(_CLOUD_SDK_MISSING_CREDENTIALS)
google.auth.exceptions.DefaultCredentialsError: Your default credentials were not found. To set up Application Default Credentials, see https://cloud.google.com/docs/authentication/external/set-up-adc for more information.

Can you make this change in another PR?

[averikitsch]

Can you move all client changes to a separate PR?