Create Bootstrap_Flex.md

[ashmeet-ashmeet]

What is the change?

Gave an intoduction to flexbox and other related terms. Described how to create one in Bootstrap and use different classes to manage layout, including responsive utilities

Related issue?

closes: #66

Checklist:

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• [x] I have followed the Code of Conduct.
• [x] I have checked there aren't other open Pull Requests for the same update/change.
• [x] I have you tested the code before submission.
• [x] My changes generates no new warnings.
• [x] I'm an HSOC22 contributor.