Add SHA checksums for "Spring Tools 4 for Eclipse" downloads

[martinlippert]

This has been reported to the STS4 issue tracker, so moving it here for discussion:
spring-projects/sts4#623

Would indeed be good if we could add those checksums somewhere on the page.

[pfurbacher]

You might want to close #726 (2017!) which asks for MD5 checksums.

For this issue, limit the checksums to at least SHA-256 or greater. Examples: Oracle provides SHA-256 for JDK downloads; Eclipse downloads provide a SHA-512 checksum.

In the meantime, I'm wondering why, some 8 months later, no action has been taken on this issue. It can't be that time consuming to add the necessary UI, the checksum generation and file i/o, and the backend code to put the filenames into the context. Isn't Spring Web supposed to make all that a snap?