* Upgrade various build and doc links to https.
Where it wasn't possible to upgrade build-time dependencies to https,
I kept http in place but used hardcoded checksums or GPG keys to ensure
that artifacts fetched over http are verified properly.
* Switch to https://apache.org.
Originally written by @AlexanderSaydakov in druid-io/druid-io.github.io#448.
I also added redirects and updated links to point to the new
datasketches-extension.html landing page for the extension, rather than to
the old page about theta sketches.