Add GitHub link roles in documentation

[btjanaka]

Description

Adds two Sphinx roles to make it easier to link to GitHub PRs and issues in the documentation. Examples are:

:pr:`123` -> Links to PR 123
:issue:`123` -> Links to issue 123

References:

• https://doughellmann.com/posts/defining-custom-roles-in-sphinx/
• https://www.sphinx-doc.org/en/master/development/tutorials/helloworld.html

TODO

• Create extension
• Replace links in history
• Replace links in rest of code

Questions

Status

• I have read the guidelines in
  CONTRIBUTING.md
• I have formatted my code using yapf
• I have tested my code by running pytest
• I have linted my code with pylint
• I have added a one-line description of my change to the changelog in
  HISTORY.md
• This PR is ready to go