feat: use standard HTTP_PROXY/HTTPS_PROXY environment variables (#499)

Update proxy configuration to use industry-standard environment variables
(HTTP_PROXY, HTTPS_PROXY, NO_PROXY) instead of custom variables.

The underlying libraries (esperanto, content-core, podcast-creator)
now automatically detect proxy settings from these standard variables.

- Bump content-core>=1.14.1 (fixes #494)
- Bump esperanto>=2.18
- Bump podcast-creator>=0.9
- Update documentation with new proxy configuration

open_notebook/utils/version_utils.py

@@ -16,6 +16,7 @@ async def get_version_from_github_async(repo_url: str, branch: str = "main") ->
     Fetch and parse the version from pyproject.toml in a public GitHub repository (async).
     """
     from urllib.parse import urlparse
+
     import httpx
     import tomli