provide and document consistent webhosted URLs for key RTX-KG2c build artifacts

[saramsey]

Every Cure has communicated via @dkoslicki that it would be helpful to them (and also, to the Penn State group of our team) if the following three KG2c build artifacts could be provided at a consistent non-login-walled webhost URL (with semantic version information in the URLs):

• Node synonymizer SQLite file
• curies_to_pmid SQLite file
• KG2c JSON-lines file

[saramsey]

It would be great if we could get this in place for the KG2c build.

It appears that this is primarily a documentation issue, and also maybe something we need to update on our KG2c roll-out issue template.