Documentation tasks for the MVP

This issue is a collection of documentation tasks that must be done as part of the MVP.

[x] Create separate docs folder and structure for the documentation within the project
[x] Installation instructions
[x] Setting up replication (including standby node) using pg_tde
[x] How to report an issue?
[x] Functionality/Features of pg_tde need to be documented
[x] Current missing features/limitations need to be documented
[x] https://github.com/Percona-Lab/postgres-tde-ext/pull/81 needs to be documented

Percona-Lab / pg_tde