infra: Add proposal for including infra documentation

infra: Add proposal for including infra documentation

This change proposes addition of infrastructure to OPNFV Documentation
on RTD.

Proposed sections are
- software-infrastructure: proposal to move documentation for OPNFV
software infrastructure such as Gerrit, Artifact Repository, Jenkins, and
so on, from OPNFV Wiki [1] to RTD.
- continuous-integration: proposal to move documentation for OPNFV
Continuous Integration from OPNFV Wiki [2] to RTD.
- cross-community-continuous-integration: proposal to move documentation
for OPNFV XCI from OPNFV Wiki [3] to RTD.

Proposal for including the documentation for hardware infrastructure is not
included in this change as there is already documentation in different place
on RTD. It will be clarified later.

[1] https://wiki.opnfv.org/display/INF/Software+Infrastructure
[2] https://wiki.opnfv.org/display/INF/Continuous+Integration
[3] https://wiki.opnfv.org/pages/viewpage.action?pageId=8687635

Change-Id: I4f22d9f22135c70b55b5e92aa3338a4c3889bee1
Signed-off-by: Fatih Degirmenci <fatih.degirmenci@ericsson.com>