docs: add --depth=2 to reduce download size (#46605)

* docs: add --depth=2 to reduce download size

* Add note to tell users about --depth=2 and manyFiles

* Fix inline code in info block

README.md

@@ -46,13 +46,18 @@ See the
 [Feature Overview](https://spack.readthedocs.io/en/latest/features.html)
 for examples and highlights.
 
-To install spack and your first package, make sure you have Python.
+To install spack and your first package, make sure you have Python & Git.
 Then:
 
-    $ git clone -c feature.manyFiles=true https://github.com/spack/spack.git
+    $ git clone -c feature.manyFiles=true --depth=2 https://github.com/spack/spack.git
     $ cd spack/bin
     $ ./spack install zlib
 
+> [!TIP]
+> `-c feature.manyFiles=true` improves git's performance on repositories with 1,000+ files.
+>
+> `--depth=2` prunes the git history to reduce the size of the Spack installation.
+
 Documentation
 ----------------