DOC: Add summary to interpolate (#37042)

* DOC: add basic description to interpolate method

This basic description can be useful for users who don't know what
interpolation is and this may help with SEO to search for more powerful
method than the `.fillna()` method.

* DOC: update fillna info to contrast interpolate

In the "See Also" section of interpolate, the fillna method sounds very
similar, but it actually is more limited. So this commit makes this
distinction more clear.

* DOC: condense interpolate method summary text

* DOC: revert description change of fillna

Author: erictleung

pandas/core/generic.py

@@ -6837,6 +6837,8 @@ def interpolate(
         **kwargs,
     ) -> Optional[FrameOrSeries]:
         """
+        Fill NaN values using an interpolation method.
+
         Please note that only ``method='linear'`` is supported for
         DataFrame/Series with a MultiIndex.