Updated build_doc.sh to build on the new release tag found

[thinksanky]

Description

• Checked out the new release tag if found and do makes docs on it.
• Rest of the logic stays same.
• Added readable comments in many places so reader can follow the logic in the code.

[thinksanky]

• Testing:
  Currently building on ubuntu. I can see the extracted tag and building it when found, but the build itself is taking too long and freezing. Retrying build again. "make docs" is taking up a lot of memory and taking too long to run.

[aaronmarkham]

I checked out your fork and ran the script using:
sudo docs/build_version_doc/build_doc.sh https://github.com/apache/incubator-mxnet-site asf-site
It created VersionedWeb folder with 1.0.0 as the default and 1.1.0 was not present.
Please provide details on how to run the script to get it to produce all of the tags.

[thinksanky]

To run locally, I had to tweak the script to set
curr_tag=1.1.0 inside the script. The {TAG} value is coming from Jenkins when the Job is run and it picks the curr_tag:5 as well. I am not very clear from Jenkins job configuration, but these two lines in the script picks the latest tag:

curr_tag=${TAG}
curr_tag=${curr_tag:5}

But for manual testing I override this value

curr_tag=${TAG}
curr_tag=${curr_tag:5}
curr_tag=1.1.0 (... for example to force build 1.1.0 and test the logic)