Release 2.1.0 docs

[j143]

Add release documentation

Step 0: Prerequisites

git clone https://github.com/apache/systemds
git clone https://github.com/apache/systemds-website

Step 1:

generate docs with as per https://github.com/apache/systemds/tree/master/docs

# In this case the release tag is 2.1.0-rc3
git checkout 2.1.0-rc3

cd systemds/docs

./updateAPI.sh

Step 2:

copy the contents to systemds-website repo

cd systemds-website

# create a new directory with release no.
mkdir content/docs/2.1.0

# copy the files in docs/_site folder to 2.1.0
cp -r ../systemds/docs/_site/* content/docs/2.1.0

Then follow, https://github.com/apache/systemds-website#updating-the-website

Website Preview at https://systemds.staged.apache.org

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Closes j143/systemds#175