### Description
renamed several repository links from `hwchase17` to `langchain-ai`.
### Why
I discovered that the README file in the devcontainer contains an old
repository name, so I took the opportunity to rename the old repository
name in all files within the repository, excluding those that do not
require changes.
### Dependencies
none
### Tag maintainer
@baskaryan
### Twitter handle
[kzk_maeda](https://twitter.com/kzk_maeda)
# Update contribution guidelines and PR template
This PR updates the contribution guidelines to include more information
on how to handle optional dependencies.
The PR template is updated to include a link to the contribution guidelines document.
