Adding versioning support to DocumentGenerator

[volkan-aslan]

I have added:

--branch : an argparse to get branch/tag name from user which documentation will be created based on it.

VERSION_PATTERN: hard coded pattern for branch/tag inputs coming from users. Only allows RX.X or RX.X.X

is_branch_name_valid(): method to check if --branch(argparse) is matching with VERSION_PATTERN

collect_existing_versions: it checks all the folders in ServicesInterfaceDocumentation repo and create a list if folder name matches with VERSION_PATTERN to find existing versions.

create_root_index: this method will create the index file which holds a dropbox and an iframe to show mkdocs files.

versions: this variable created to check if we are overriding an existing version or creating a new one

sorted_versions: variable to fill dropbox values

Also:
site_name changed from "Documentation" to branch_name to understand which version is currently shown on webpage.

if branch is given as RX.X, script automatically changes . with _ to download correct repository.

Instead of using mkdocs gh-deploy, we now use mkdocs build -d{branch_name} to create files locally in a folder
and then adding, commiting and pushing index.html and the folder that is holding mkdocs files to remote.