[#166] Add page describing data objects (4-2-stable)

The following sections were added:

Data Objects and Replicas
Logical Locking
High-Level Operations
- Put
- Get
- Copy
- Replicate
- Phymv
- Trim
- Unlink
- Rename
Description of R_DATA_MAIN

Each operation has been accompanied by a diagram to depict what is happening.

A deeper dive into the implementations for the various operations as well as our POSIX operations should be added in the Doxygen for the API implementations.

irods / irods_docs

[#166] Add page describing data objects (4-2-stable) #168