build: stop shipping docs/ inside the npm package (#3811)

tools/build.ts copied the whole docs/ tree into the publish output,
so every release carried 53 Markdown and image files, about 1 MB
unpacked and 42 percent of the compressed tarball, that nothing in
the package reads. The copy dates from the 6.0.0 build script
(#2282).

Shipped docs freeze at the installed version while the GitHub copies
move on, and docs/firebase.json inside node_modules has been mistaken
for a config file to point a deploy at. README.md still ships.

Measured on the 22.0.0 build at f95d598: 175 to 122 files, 4,694,110
to 3,663,246 bytes unpacked, 2,021,273 to 1,173,999 bytes compressed.

Author: armando-navarro

tools/build.ts

@@ -514,7 +514,6 @@ async function buildLibrary() {
   await Promise.all([
     copy(join(process.cwd(), '.npmignore'), dest('.npmignore')),
     copy(join(process.cwd(), 'README.md'), dest('README.md')),
-    copy(join(process.cwd(), 'docs'), dest('docs')),
     compileSchematics(),
     replacePackageCoreVersion(),
   ]);