Initial installation doc refactor

These changes work to improve GitHub CLI users experience by bringing:

- clarity between official and unofficial installation methods
- insight into unofficial installation methods
- consistency across supported operating systems

docs/install_source.md

@@ -0,0 +1,59 @@
+# Installation from source
+
+1. Verify that you have Go 1.24+ installed
+
+   ```sh
+   $ go version
+   ```
+
+   If `go` is not installed, follow instructions on [the Go website](https://golang.org/doc/install).
+
+2. Clone this repository
+
+   ```sh
+   $ git clone https://github.com/cli/cli.git gh-cli
+   $ cd gh-cli
+   ```
+
+3. Build and install
+
+   #### Unix-like systems
+   ```sh
+   # installs to '/usr/local' by default; sudo may be required, or sudo -E for configured go environments
+   $ make install
+
+   # or, install to a different location
+   $ make install prefix=/path/to/gh
+   ```
+
+   #### Windows
+   ```pwsh
+   # build the `bin\gh.exe` binary
+   > go run script\build.go
+   ```
+   There is no install step available on Windows.
+
+4. Run `gh version` to check if it worked.
+
+   #### Windows
+   Run `bin\gh version` to check if it worked.
+
+## Cross-compiling binaries for different platforms
+
+You can use any platform with Go installed to build a binary that is intended for another platform
+or CPU architecture. This is achieved by setting environment variables such as GOOS and GOARCH.
+
+For example, to compile the `gh` binary for the 32-bit Raspberry Pi OS:
+```sh
+# on a Unix-like system:
+$ GOOS=linux GOARCH=arm GOARM=7 CGO_ENABLED=0 make clean bin/gh
+```
+```pwsh
+# on Windows, pass environment variables as arguments to the build script:
+> go run script\build.go clean bin\gh GOOS=linux GOARCH=arm GOARM=7 CGO_ENABLED=0
+```
+
+Run `go tool dist list` to list all supported values of GOOS/GOARCH.
+
+Tip: to reduce the size of the resulting binary, you can use `GO_LDFLAGS="-s -w"`. This omits
+symbol tables used for debugging. See the list of [supported linker flags](https://golang.org/cmd/link/).