Add initial offline build instructions (#206)

Release v0.0.5 is the first release which contains the artifact
(artifact uploaded manually, future releases will directly add it from
CI)


Testing:

```shell
# in the servo repository create a new clean worktree based on latest main
git worktree add ../servo-vendor-testing
cd ../servo-vendor-testing
# Create a vendored tar-ball (which would be provided by a servo release)
./etc/vendor_servo.py
mkdir ../offline-servo
cd ../offline-servo
# Extracts the tarball into the current directory
tar -xf ../servo-vendor-testing/servo.tar.gz
uv sync
# Now disconnect the internet and build (assuming bootstrap was run before on the machine)
./mach build
```

---------

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>
Signed-off-by: Jonathan Schwender <55576758+jschwe@users.noreply.github.com>
Co-authored-by: Josh Matthews <josh@joshmatthews.net>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

Author: 3 people

src/SUMMARY.md

@@ -18,6 +18,7 @@
   - [WSL](building/wsl.md)
   - [Android](building/android.md)
   - [OpenHarmony](building/openharmony.md)
+  - [Building Offline](building/building-offline.md)
 - [General Troubleshooting](building/general-troubleshooting.md)
 
 # Contributing

src/building/offline-builds.md

@@ -0,0 +1,46 @@
+# Building Offline
+
+Servo releases provide a `servo-<release-tag>-src-vendored.tar.gz` artifact which contains Servo's source code and all Rust dependencies vendored.
+The `.cargo/config.toml` file in the tarball contains the necessary configuration to build Servo offline using the vendored dependencies.
+
+## Linux
+
+Please view the [Linux build instructions](./linux.md) for more on installing the required build dependencies.
+We generally recommend using `./mach build` to build Servo, however that does require `uv` to be installed and the Python dependencies to be synced via `uv sync` in advance (otherwise `./mach` will try to access the network to setup the python environment). 
+```shell
+# online pre-build environment (e.g. building a docker container)
+uv sync
+# offline build environment
+./mach build --profile=production --frozen
+```
+
+In cases where this is difficult, you can also build Servo using `cargo`, in which case any recent Python version (>= 3.11) should be sufficient (but this is not tested in CI).
+Note that `./mach build` will enable the `media-gstreamer` feature by default - When using `cargo` you need to enable this feature manually, and set any gstreamer-related environment variables as well.
+The required environment variables for the gstreamer feature are not documented here, but documentation improvements based on our `./mach build` code are welcome.
+
+```shell
+# Build an offline release build with the production profile.
+cargo build --profile=production --frozen
+```
+
+
+
+
+## Windows and macOS
+
+On Windows and macOS, `./mach bootstrap` will download additional dependencies necessary to build Servo.
+These are currently not provided in the tarball. 
+If there is interest in offline builds for these platforms, contributions are welcome (but reach out first on Zulip or via GitHub issues).
+
+
+## Prebuilt SpiderMonkey Artifacts
+
+Online builds use prebuilt SpiderMonkey artifacts by default, hosted on [servo/mozjs](https://github.com/servo/mozjs)'s GitHub releases.
+If you want to simplify or speed-up your build environment, you can pre-download these artifacts yourself and use `MOZJS_ARCHIVE=path/to/libmozjs.tar.gz` to use the prebuilt artifacts in offline builds.
+You can verify the integrity of the downloaded artifacts by using [GitHub attestions] with the `gh` tool:
+
+```shell
+gh attestion verify path/to/libmozjs.tar.gz -R servo/mozjs
+```
+
+[GitHub attestions]: https://docs.github.com/en/actions/how-tos/secure-your-work/use-artifact-attestations/use-artifact-attestations