15.2/relnotes: prepare release documentation for 15.2-BETA1 publication

[vladlen]

Summary

Historically, release notes were not prepared for BETA1, so users checking
the documentation right after the announcement found it empty (confirmed
by page-view stats) and asked where the release documentation was.
To avoid this, I prepared the release documentation to be ready by the BETA1
release date, so it can be reviewed and commented on early.

Note: this is not the release documentation in BETA1 — it is BETA1 of the release
documentation.Upgrade instructions for BETA1 are in the BETA1 announcement,
not here.

I will land it to make all changes visible.