[SPARK-3430] [PySpark] [Doc] generate PySpark API docs using Sphinx

Using Sphinx to generate API docs for PySpark.

requirement: Sphinx

```
$ cd python/docs/
$ make html
```

The generated API docs will be located at python/docs/_build/html/index.html

It can co-exists with those generated by Epydoc.

This is the first working version, after merging in, then we can continue to improve it and replace the epydoc finally.

Author: Davies Liu <davies.liu@gmail.com>

Closes #2292 from davies/sphinx and squashes the following commits:

425a3b1 [Davies Liu] cleanup
1573298 [Davies Liu] move docs to python/docs/
5fe3903 [Davies Liu] Merge branch 'master' into sphinx
9468ab0 [Davies Liu] fix makefile
b408f38 [Davies Liu] address all comments
e2ccb1b [Davies Liu] update name and version
9081ead [Davies Liu] generate PySpark API docs using Sphinx

1 files changed, 27 insertions, 0 deletions

diff --git a/python/docs/epytext.py b/python/docs/epytext.py
new file mode 100644
index 0000000000..61d731bff5
--- /dev/null
+++ b/python/docs/epytext.py
@@ -0,0 +1,27 @@
+import re
+
+RULES = (
+    (r"<[\w.]+>", r""),
+    (r"L{([\w.()]+)}", r":class:`\1`"),
+    (r"[LC]{(\w+\.\w+)\(\)}", r":func:`\1`"),
+    (r"C{([\w.()]+)}", r":class:`\1`"),
+    (r"[IBCM]{(.+)}", r"`\1`"),
+    ('pyspark.rdd.RDD', 'RDD'),
+)
+
+def _convert_epytext(line):
+    """
+    >>> _convert_epytext("L{A}")
+    :class:`A`
+    """
+    line = line.replace('@', ':')
+    for p, sub in RULES:
+        line = re.sub(p, sub, line)
+    return line
+
+def _process_docstring(app, what, name, obj, options, lines):
+    for i in range(len(lines)):
+        lines[i] = _convert_epytext(lines[i])
+
+def setup(app):
+    app.connect("autodoc-process-docstring", _process_docstring)