documentation

[DEV-2097] improved history primary key upgrade docs

Merge in WEB/documentation from feature/DEV-2097-6.0-D to release/6.0

* commit 'f9495deec89f03fa283c267a5fb23a3a5937a3cb': (28 commits)
  updated block IDs in db_primary_keys.md
  [DEV-2097] added few more changes
  [DEV-2097] Few more fixes for DB primary keys
  [DEV-2097] more changes to db_primary_keys.md
  [DEV-2097] rewriting db_primary_keys.md
  db_primary_keys.md changed size for all headers
  [DEV-2097] removed tsdb version, fixed anchor link
  [DEV-2097] made oracle example consistent
  [DEV-2097] fixed anchor link for postgres
  [DEV-2097] added periods in important notes
  [DEV-2097] various doc fixes
  [DEV-2097] various doc fixes
  [DEV-2097] fixed sql indent
  [DEV-2097] make pgsql and tsdb manual more precise and separate
  [DEV-2097] doc fixes
  [DEV-2097] doc fixes
  [DEV-2097] doc fixes
  [DEV-2097] doc fixes
  [DEV-2097] tiny fix
  [DEV-2097] tiny fix
  ...