buildsys: docs: restore clean target

[leleliu008]

docs clean target was removed in #4465

We expect that clean target will remove _build/ _ext/__pycache__ directories. sphinx-build -M clean command won't do this.

If users don't want to generate docs, it will fail while executing make clean if sphinx-build is not installed.

It is unreasonable to force users to install Sphinx, as they may have only executed make clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.98%. Comparing base (069715e) to head (d47fc5d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4474   +/-   ##
=======================================
  Coverage   85.98%   85.98%           
=======================================
  Files         255      255           
  Lines       63940    63940           
=======================================
  Hits        54978    54978           
  Misses       8962     8962           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[masatake]

Looks good. However, I want to make changes to the docs/Makefile minimal.

I found a way to externalize your change:

diff --git a/docs/Makefile b/docs/Makefile
index d4bb2cbb9..ba3d446ce 100644
--- a/docs/Makefile
+++ b/docs/Makefile
@@ -18,3 +18,5 @@ help:
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
    @$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+
+include ext.mak
diff --git a/docs/ext.mak b/docs/ext.mak
new file mode 100644
index 000000000..69a29f5d3
--- /dev/null
+++ b/docs/ext.mak
@@ -0,0 +1,8 @@
+.PHONY: ext.mak
+
+EXTDIR        = _ext
+
+.PHONY: clean
+
+clean:
+   rm -rf $(BUILDDIR) $(EXTDIR)/__pycache__