docs+lnrpc: rename rpc.proto to lightning.proto

To avoid a naming conflict with etcd, we rename our very generic
rpc.proto to lightning.proto to match the service name that's declared
within. This will break many external tutorials and possibly also our
API docs but the change needs to be done eventually.

docs/grpc/c#.md

@@ -23,7 +23,7 @@ Create a folder `Grpc` in the root of your project and fetch the lnd proto files
 
 ```shell
 ⛰  mkdir Grpc
-⛰  curl -o Grpc/rpc.proto -s https://raw.githubusercontent.com/lightningnetwork/lnd/master/lnrpc/rpc.proto
+⛰  curl -o Grpc/lightning.proto -s https://raw.githubusercontent.com/lightningnetwork/lnd/master/lnrpc/lightning.proto
 ```
 
 Install `Grpc.Tools`, `Google.Protobuf`, `Grpc.Core` using NuGet or manually with `dotnet add`:
@@ -34,11 +34,11 @@ Install `Grpc.Tools`, `Google.Protobuf`, `Grpc.Core` using NuGet or manually wit
 ⛰  dotnet add package Grpc.Core
 ```
 
-Add the `rpc.proto` file to the `.csproj` file in an ItemGroup. (In Visual Studio you can do this by unloading the project, editing the `.csproj` file and then reloading it)
+Add the `lightning.proto` file to the `.csproj` file in an ItemGroup. (In Visual Studio you can do this by unloading the project, editing the `.csproj` file and then reloading it)
 
 ```xml
 <ItemGroup>
-   <Protobuf Include="Grpc\rpc.proto" GrpcServices="Client" />
+   <Protobuf Include="Grpc\lightning.proto" GrpcServices="Client" />
 </ItemGroup>
 ```