doc: add table of contents in BUILDING.md

PR-URL: #23147
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

BUILDING.md

@@ -8,6 +8,43 @@ If you can reproduce a test failure consistently, search for it in the
 [Node.js issue tracker](https://github.com/nodejs/node/issues) or
 file a new issue.
 
+## Table of Contents
+
+* [Supported platforms](#supported-platforms)
+  * [Input](#input)
+  * [Strategy](#strategy)
+  * [Supported platforms](#supported-platforms-1)
+  * [Supported toolchains](#supported-toolchains)
+    * [Unix](#unix)
+    * [AIX](#aix)
+    * [Windows](#windows)
+    * [OpenSSL asm support](#openssl-asm-support)
+* [Building Node.js on supported platforms](#building-nodejs-on-supported-platforms)
+  * [Unix/macOS](#unixmacos)
+    * [Prerequisites](#prerequisites)
+    * [Building Node.js](#building-nodejs-1)
+    * [Running Tests](#running-tests)
+    * [Building the documentation](#building-the-documentation)
+    * [Building a debug build](#building-a-debug-build)
+  * [Windows](#windows-1)
+  * [Android/Android-based devices (e.g. Firefox OS)](#androidandroid-based-devices-eg-firefox-os)
+  * [`Intl` (ECMA-402) support](#intl-ecma-402-support)
+    * [Default: `small-icu` (English only) support](#default-small-icu-english-only-support)
+    * [Build with full ICU support (all locales supported by ICU)](#build-with-full-icu-support-all-locales-supported-by-icu)
+      * [Unix/macOS](#unixmacos-1)
+      * [Windows](#windows-2)
+    * [Building without Intl support](#building-without-intl-support)
+      * [Unix/macOS](#unixmacos-2)
+      * [Windows](#windows-3)
+    * [Use existing installed ICU (Unix/macOS only)](#use-existing-installed-icu-unixmacos-only)
+    * [Build with a specific ICU](#build-with-a-specific-icu)
+      * [Unix/macOS](#unixmacos-3)
+      * [Windows](#windows-4)
+* [Building Node.js with FIPS-compliant OpenSSL](#building-nodejs-with-fips-compliant-openssl)
+* [Building Node.js with external core modules](#building-nodejs-with-external-core-modules)
+  * [Unix/macOS](#unixmacos-4)
+  * [Windows](#windows-5)
+
 ## Supported platforms
 
 This list of supported platforms is current as of the branch/release to