Add pathvalue example to README and implement PathValue handler.

[catatsuy]

This pull request includes updates to the chi examples, specifically adding a new example that demonstrates the usage of PathValue for retrieving URL parameters. The most important changes include the addition of a new example in the _examples/README.md file and the implementation of the pathvalue example in a new Go file.

New example addition:

• _examples/README.md: Added a new entry for the pathvalue example, which demonstrates the usage of PathValue for retrieving URL parameters.

Implementation of the pathvalue example:

• _examples/pathvalue/main.go: Created a new Go file that includes a router setup with a handler to retrieve and respond with a URL parameter using PathValue.

refs: #984