Add build dependencies and other local packages needed (Ubuntu only).
Fix some weird formatting of psql commands due to `sh` syntax
highlighting.
Improve test directions, so pytest doesn't scan the whole tree.
Drop description of the integration_tests directory since it's on its
way out.
* Add ancestor_id to pg_list->branch_list output of pageserver.
* Display branching point (LSN) for each non-root branch.
* Add tests for `zenith branch`.
Currently, the tests don't work in parallel. The tests try to create a
data directory in the same path and clash with each other. That should be
fixed, but for now, let's at least fix the instructions on how it works
now.
