Update Skylark documentation
Update Skylark documentation

- Add debugging tips
- Give more visibility to tools/build_defs/g3doc/
- Mention Standalone Skylark
- Remove some mentions of build extensions

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MOS_MIGRATED_REVID=137175010

davidzchen/bazel-website,bazelbuild/bazel-website,davidzchen/bazel-website,bazelbuild/bazel-website,bazelbuild/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website