docs(readme): build from the signed release archive (#6315)

* docs(readme): build from the signed release archive

Users building Maka from source were sent to a Git checkout of main, which is unreleased code. Point them at the signed source archive of the latest Apache release instead, with the download, signature and checksum steps, and keep the Git checkout as the contributor development path.

This follows the README change promised to the IPMC during the 0.2.2 RC2 vote.

Generated-by: Claude Code

* docs(readme): reuse release download and verification guide

Generated-by: Codex

README.md

@@ -58,7 +58,7 @@ The [website](https://maka.apache.org/en/) walks through one turn of the log and
 
 **Development testing**: To contribute or help test unreleased changes, see [CONTRIBUTING](./CONTRIBUTING.md) and the [development mailing list](https://lists.apache.org/list.html?dev@maka.apache.org). Development builds are not approved Apache releases.
 
-**Build from source**: To compile and run Desktop, the TUI, or the CLI directly from a source checkout, see the [Build from source](#build-from-source) section below.
+**Build from source**: To compile and run Desktop, the TUI, or the CLI, see the [Build from source](#build-from-source) section below.
 
 ## Build from source
 
@@ -66,10 +66,21 @@ The [website](https://maka.apache.org/en/) walks through one turn of the log and
 
 - Node.js 22.19 or newer on the 22 line, or Node.js 24 or newer (CI uses Node.js 24; Node.js 23 is not supported);
 - npm (the lockfile and scripts use npm; the current `packageManager` is npm 11);
-- Git;
+- GnuPG to verify a release archive, or Git for contributor development;
 - `ripgrep`, used by Runtime's `Grep` tool.
 
-### Start Desktop
+### Build an Apache release
+
+Download and verify the signed source archive using the [downloads page](https://maka.apache.org/en/downloads/). Extract it, then build and start Desktop from the extracted directory:
+
+```sh
+npm ci
+npm run dev:full
+```
+
+### Contributor development
+
+For unreleased development builds, start from a Git checkout:
 
 ```sh
 git clone https://github.com/apache/maka.git