Add a filter for specifications

[oicheryl]

Goal: Make it easy for government procurement to refer to product neutral requirements.

Description: Government procurement teams cannot require individual tools/projects ("use Envoy") and instead need to cite APIs/specifications ("compatible with the Container Runtime Interface").

Request: Could we add a filter for specifications to the left hand side of l.cncf.io, so they can easily find all the CNCF-approved specifications?