Clean up known issues list in app/README.md and improve documentation

• Clean up known issues list in app/README.md after merge latest integration with search
• Improve documentation and remove LEGACY_README.md.
• Be sure to include details like:

Partition bundle contains _.manifest_ with following contents:

{ "roots": ["osdu/partiton/data_partition_id"] }

To define a policy within a data partition, one must include a namespace that is specific for the given data partition. For example, _dataauthz_ policy for data partition _data_partition_id_ can look like this:

package osdu.partition["dp1-100"].dataauthz

import data.osdu.instance.dataauthz

records := data.osdu.instance.dataauthz.records