Since we always forget to update the documentation lets check this
automatically.

* cmd: include replay command in docs even if rr is not installed.

* path/to/binary is the required argument

Replaces #847

Implements #727

New API version with better backwards compatibility plus mechanism to
select the API version that a headless instance should use.

Adds service/test/cmd/typecheckrpc.go to type check the RPC interface.

Going forward, all documentation should be placed in the Documentation
directory in the root of the project. This switch allows maintainers to
approve updates to documentation before they are committed, as opposed
to the pre-existing wiki which anybody could modify.

Currently the Documentation directory includes docs on building, usage,
and minimal docs around the API. This is just the initial commit, and
documentation will continue to improve over time.

Some changes have been made (and will continue to be made) to `cmd/dlv` to
ensure we can auto-generate documentation for all commands from the
newly provided script `scripts/gen-usage-docs.go`, which can be invoked
via `go run scripts/gen-usage-docs.go`.

Additionally, version has been split into its own package. This was a
bit of housekeeping related to the changes made the `cmd/dlv`.