fix content-type for zstd files, additionally build & serve gzip compressed rustdoc json

[syphar]

• we previously did set the wrong content type for the zstd file , that's fixed
• I needed a storage method to upload without compression, so I can compress myself before uploading
• I also don't want Content-Encoding to be set, so the files are offered as downloads, not rendered in the browser

Apart from that, most of the code is:

• adding gzip compression
• additionally building & storing gzip
• supporting a .zst or .gz suffix on the URLs.
• still supporting a fallback to the old files we have built until now.