stop hosting at https://cap-lab.bio/developing-with-qiime2

[gregcaporaso]

Now that we're set up with Read The Docs (RTD; https://develop.qiime2.org), we can stop hosting through GitHub pages. Ideally we'll have a index.html page or a permanent redirect at the old location (https://cap-lab.bio/developing-with-qiime2) that redirects to the new canonical URL (https://develop.qiime2.org).

• Delete the requirements.txt file as part of the PR for this issue, as that isn't needed by RTD (which uses the environment.yml file).
• Delete the GitHub Actions workflow.