[SYSTEMDS-3047] Add release documentation 2.1.0

[j143]

⚠please do not merge via github UI, rebase and merge option is changing the commit sha.
We need to push change via command line. asf-site and asf-staging are to be in sync!

Website preview: https://systemds.staged.apache.org/docs/2.1.0/index

Companion to #94

update instructions

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

Merge Instructions:

git checkout asf-staging
git rebase asf-staging asf-site

Push the changes to your personal fork, make sure that the commit sha is fine

git push <personal-fork> asf-site

Now,

git push apache asf-site

If anyone is interested in reviewing this PR. Please go ahead.

[j143]

cc @phaniarnab @Baunsgaard