Improving `generate.py`

[mdrpanwar]

I just used generate.py to create a notebook and noticed that some improvements could be made to the same. Through this PR, I propose the following modifications / additions to generate.py:

• removed the need to input the implicit .ipynb extension along with file names.
• added sidebar variable to the frontmatter with the default value of true.
• added <!--more --> tag in frontmatter and improved the description of summary variable and <!--more --> tag.
• modified the README.md and usage comments of generate.py.

[mdrpanwar]

@Priyansi Please let me know if this helps improve the generate.py or if any changes are needed. Thanks.