chore: Update docs for building Zebra (#8315)

* Update build instructions with scanning enabled

* Remove `pkg-config` from build deps in README

I tried compiling Zebra with `--all-features` on a Linux machine without
`pkg-config` or `pkfgconf` (which is an alternative to `pkg-config`),
and it worked.

* Add `protoc` to build deps in README

README.md

@@ -61,8 +61,8 @@ For more information, read our [Docker documentation](https://zebra.zfnd.org/use
 ### Building Zebra
 
 Building Zebra requires [Rust](https://www.rust-lang.org/tools/install),
-[libclang](https://clang.llvm.org/doxygen/group__CINDEX.html),
-[pkg-config](http://pkgconf.org/), and a C++ compiler.
+[libclang](https://clang.llvm.org/doxygen/group__CINDEX.html), and a C++
+compiler.
 
 Zebra is tested with the latest `stable` Rust version. Earlier versions are not
 supported or tested. Any Zebra release can start depending on new features in the
@@ -86,7 +86,14 @@ Below are quick summaries for installing the dependencies on your machine.
      package manager. Typical names are `libclang`, `libclang-dev`, `llvm`, or
      `llvm-dev`.
    - **clang** or another C++ compiler: `g++` (all platforms) or `Xcode` (macOS).
-   - **pkg-config**
+   - **[`protoc`](https://grpc.io/docs/protoc-installation/)**
+
+> [!NOTE]
+> Zebra uses the `--experimental_allow_proto3_optional` flag with `protoc`
+> during compilation. This flag was introduced in [Protocol Buffers
+> v3.12.0](https://github.com/protocolbuffers/protobuf/releases/tag/v3.12.0)
+> released in May 16, 2020, so make sure you're not using a version of `protoc`
+> older than 3.12.
 
 </details>
 
@@ -97,7 +104,7 @@ Below are quick summaries for installing the dependencies on your machine.
 </summary>
 
 ```sh
-sudo pacman -S rust clang pkgconf
+sudo pacman -S rust clang protobuf
 ```
 
 Note that the package `clang` includes `libclang` as well as the C++ compiler.

book/src/user/shielded-scan.md

@@ -15,7 +15,7 @@ keys for experimental use or publicly known keys.
 
 ## Build & Install
 
-Use [Zebra 1.5.0](https://github.com/ZcashFoundation/zebra/releases/tag/v1.5.0)
+Use [Zebra 1.6.0](https://github.com/ZcashFoundation/zebra/releases/tag/v1.6.0)
 or greater, or the `main` branch to get the latest features, and enable the
 `shielded-scan` feature during the build. You can also use Rust's `cargo` to
 install the latest release: