Release v0.6.4

[choldgraf]

This is a meta-issue to track the release of the new version of Jupyter Book.

I don't think that we've made any breaking changes, so I was thinking of making this a minor release. @emdupre what do you think?

To do

• Finalize issues to include before this release (see https://github.com/jupyter/jupyter-book/milestone/4)
• Create a release changelog
• Create the release
• Announce the release

Release ToDo

• Ensure that there aren't any extra issues to tackle in
  the milestone for the next release

• Label any major bug fixes, enhancements, maintenance, and documentation improvements in the merged PRs

• Ensure that the Jupyter Book version number
  is correct, and remove the dev0 part of the version number.
  Make a PR with the new number and merge into master.

• Delete the extra files we don't want to include in the package, currently this is
  everything in::

  • jupyter_book/book_template/_site
  • jupyter_book/book_template/_build

• Create a new distribution for Jupyter Book by
  following the twine release instructions

• Confirm that the new version of Jupyter Book is posted to pypi

• Create a new release on GitHub.

  Call the release simply the version of Jupyter Book you are releasing. Leave the description blank, we'll
  fill it in later.
  

• Bump the Jupyter Book version number to
  the next minor (or major) release and append dev0 to the end.

• Generate a changelog using the github-activity package.

  • See here for github-activity installation instructions.

  • Run the generator with this command:

    github-activity jupyter/jupyter-book --since <<latest-tag>> --until <<new-release-tag>> --auth <<your-github-api-token>> -o tmp.md
    

  • It will create a markdown changelog since the last jupyter-book release, and put it
    in tmp.md. Copy the sections about pull requests and add it
    to CHANGELOG.md.
    Ensure that the new version looks correct, then commit it to master.

  • Create a release summary that describes the general changes that have been made,
    add it to the changelog generated by github-activity. Add this to the new section of CHANGELOG.md`.

  • Commit the new changelog and merge it into master.

• Copy the resulting markdown for the latest release into the GitHub release you just created.

• Celebrate! You've just released a new version of Jupyter Book!