Consoliate Installation Documentation

[johnwhumphreys]

The installation instructions are duplicated and have slightly different content in:

The main README.md (larger, has mac, ubuntu, etc instructions too):

• https://github.com/meta-pytorch/monarch/blob/main/README.md#installation

and the dedicated install guide (smaller).

• https://github.com/meta-pytorch/monarch/blob/main/docs/source/installation.md

We should consolidate these guides and decide what to keep between them.