GitHub action to automate resource index rebuilding

[jameshadfield]

GitHub action to rebuild the resource collection index. ~Action has not been tested yet.~ Action ran successfully and the uploaded index was tested locally.

[jameshadfield]

The commits in this PR have been combined and shifted to #719. The changes since this PR are:

• Action filename renamed as suggested
• The added documentation slightly changed after editing
• The indexer is now run with additional arguments --resourceTypes dataset --collections core as that's the only data that the server will currently use for the functionality introduced in #719. These arguments will be removed as we expand the functionality of nextstrain.org to allow further information in the index to be used.