SPARK-971 [DOCS] Link to Confluence wiki from project website / documentation

This is a trivial change to add links to the wiki from `README.md` and the main docs page. It is already linked to from spark.apache.org.

Author: Sean Owen <sowen@cloudera.com>

Closes #3169 from srowen/SPARK-971 and squashes the following commits:

dcb84d0 [Sean Owen] Add link to wiki from README, docs home page
SPARK-1344 [DOCS] Scala API docs for top methods

Use "k" in javadoc of top and takeOrdered to avoid confusion with type K in pair RDDs. I think this resolves the discussion in SPARK-1344.

Author: Sean Owen <sowen@cloudera.com>

Closes #3168 from srowen/SPARK-1344 and squashes the following commits:

6963fcc [Sean Owen] Use "k" in javadoc of top and takeOrdered to avoid confusion with type K in pair RDDs
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