Add release notes for all recent releases to the documentation page

[paulblum]

Closes issue #38.

• Adds new pages to the site that contain the release notes of all recent versions of Cantera. These pages are linked to in a new card titled "Release Notes" which has been added to the Documentation page.
• Adds a command plugin called new_release. When called, this plugin creates a new page that contains the release notes from the latest version of Cantera on GitHub. A link to the new page is then added to the Release Notes card automatically.

Notes:

• requirements.txt has been updated, please reinstall before building the site.
• This pull request adds PyMdown's MagicLink extension, enabling auto-linking and shorthand display formats for web addresses and Github references for all Markdown files. See the full documentation for MagicLink here: https://facelessuser.github.io/pymdown-extensions/extensions/magiclink/