Document VPR placement constraints

[sfkhalid]

Description

Added a documentation page to describe how a user can run VPR with placement constraints.

Related Issue

Related to the features add for issue #932.

[vaughnbetz]

@sfkhalid : please address the comments before we meet tomorrow so we can get this one merged.

[vaughnbetz]

@kgugala @mithro : seems the google cloud is failing on most or all of the tests that are going to it on this PR. I think the tests that passed are those run on github's resources. Error is the usual resource one:
ERROR: (gcloud.beta.compute.instances.create) Could not fetch resource:
14:44:51 | - The zone 'projects/***/zones/us-west1-a' does not have enough resources available to fulfill the request. '(resource type:compute)'.
Can we move to a different cloud region or make some other setting change to get this going? At this point the google cloud seems significantly less reliable than kokoro was, unfortuntely.

[vaughnbetz]

Changes look good; this is ready to merge when CI goes green (or I could merge without CI as Sarah tells me the documentation builds).

[vaughnbetz]

Read the docs test passed and all the failures look like unrelated google cloud issues; merging.