docs(build): document SDK and VM build workflow

[VicMisael]

Description

This PR adds a build guide for compiling the TotalCross SDK first and then
building the TotalCross VM and launcher.

The documentation was created while revisiting the project after several
years. It also provides a clearer onboarding path for new contributors,
including prerequisites, generated artifacts, platform-specific instructions,
and troubleshooting for common build failures.

Related Issue

Not applicable. This is a documentation-only improvement and does not
introduce a feature or fix a runtime bug.

Motivation and Context

The repository contains the required build configuration, but the complete
SDK-to-VM workflow was spread across Gradle files, CMake configuration,
platform-specific documentation, scripts, and CI workflows.

The new guide documents:

• required build tools and dependencies;
• why the SDK must be built before the VM;
• how SDK and runtime TCZ files are generated;
• how to build libtcvm and the native launcher;
• how to assemble a runnable application directory;
• Android and iOS build commands;
• Gradle log locations and verbosity options; and
• troubleshooting for JDK 21 and missing SDL2 dependencies.

This should make it easier for returning and first-time contributors to
understand and reproduce the build process.

Benefited Devices

• Device: development workstations
• OS: Linux and macOS; Android and iOS build instructions are also included

How Has This Been Tested?

This is a documentation-only change for TotalCross 7.2.2.

The instructions were checked against the current repository configuration,
including:

• TotalCrossSDK/build.gradle;
• TotalCrossVM/CMakeLists.txt;
• Android Gradle configuration;
• iOS build scripts;
• GitHub Actions build workflows; and
• SDK Gradle build logs.

The following documentation validations were run:

python3 scripts/validate-copyright-headers.sh --files BUILDING.md
git diff --check -- BUILDING.md

Both commands passed.

A full SDK, native VM, Android, and iOS build matrix was not run as part of
this documentation-only change. These builds require dependency downloads and
platform-specific toolchains.

Tested Devices

• Device: development workstation
• OS: Linux

No physical Android or iOS devices were used because this PR only changes
documentation.

Screenshots or Videos

Not applicable.