docs: clarify PHP library path for source builds

[ousamabenyounes]

Fix #601.

Changes

• Add the selected PHP prefix library directory to the CGO_LDFLAGS example used with xcaddy, so the linker can find FrankenPHP's hard-coded -lphp when libphp is installed outside the default linker paths.
• Mention that versioned php-config commands must be used consistently in the cgo flags.
• Use the repository go.sh wrapper for the direct go build example, now that it owns the default build tags and php-config handling.

Validation

Verified in Docker with dunglas/frankenphp:1.12.6-builder-php8.5, Go 1.26.5 from the official Go image, and pinned xcaddy tag v0.4.5.

The build used the documented values:

CGO_CFLAGS="$(php-config --includes)"
CGO_LDFLAGS="$(php-config --ldflags) -L$(php-config --prefix)/lib $(php-config --libs)"
XCADDY_GO_BUILD_FLAGS="-ldflags=\"-w -s\" -tags=nobadger,nomysql,nopgx"
xcaddy build --output /tmp/frankenphp-doc-verify \
  --with github.com/dunglas/frankenphp=/src \
  --with github.com/dunglas/frankenphp/caddy=/src/caddy

Result:

Build complete: /tmp/frankenphp-doc-verify
/tmp/frankenphp-doc-verify version
v2.11.4 h1:XKxkMTgNSizEvKG6QHue6cAsFOteU2qA61w2tKkCWi0=

Also ran git diff --check.