No single source of truth for development documentation

**Describe the bug**
Both the wiki and the manual contain some documentation of development process and requirements of QMCPACK

**To Reproduce**
Steps to reproduce the behavior:
Read one read the other both are incomplete, sometimes they probably don't agree.

**Expected behavior**
I expect we'll all read and understand the same development documents.

We need to chose one place. The other can have a link, it will be alright.

## In favor of manual
* tracks the commits of the code.

## In favor of the wiki
* Its easy to consult and edit.