Commit edba2e97 renamed pageserver/README to pageserver/README.md, but
forgot to update links to it. Fix.
Rename libs/postgres_ffi/README and safekeeper/README files to also
have the the .md extension, so that github can render them nicely.
Quote ascii-diagram in safekeeper/README.md so that it renders
correctly.
Remove when it is consumed by all of 1) pageserver (remote_consistent_lsn) 2)
safekeeper peers 3) s3 WAL offloading.
In test s3 offloading for now is mocked by directly bumping s3_wal_lsn.
ref #1403