Doc: mention profiling and specification.
Also clean up the rest of the page.
MOS_MIGRATED_REVID=147353770
@@ -15,3 +15,12 @@ Read the [concepts](concepts.md) behind Skylark and try the
[standard library](lib/globals.html).
-->
+
+## How can I profile my code?
+
+```shell
+$ bazel build --nobuild --profile=/tmp/prof //path/to:target
+$ bazel analyze-profile /tmp/prof --html --html_details
+```
+
+Then, open the generated HTML file (`/tmp/prof.html` in the example).
