Add pull request and issue templates

Adding issue templates and pull request template will enhance the workflow of the repository.

Use the new form of ISSUE_TEMPLATE i.e. making a folder ISSUE_TEMPLATE in `.github` and adding different types of `.md` files for different issues like `bug.md`, `documentation.md` and `feature.md`. A contributor can choose the type of templates whenever they make an issue.

Reference: https://docs.github.com/en/github/building-a-strong-community/about-issue-and-pull-request-templates