WIP: Transcode introduction

This file incorporates the changes suggested, along with explaining transcoding more properly and describing the four scenarios.

I want to ask a couple of things for this:

1. Should I add a part in the start for explaining 'codec' and 'muxer', or should it be done in the glossary?
2. We discussed that elementary streams are to be talked about in "under the hood" section as it is more advanced. Hence, I have called them "raw streams" in the diagram. Should I use the term "elementary streams"?

mentioned in issue #3

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• 73d8f69a - Updating the scenarios

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• c094f46c - elaborating the scenarios

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mentioned in issue #6

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• 5ac58198 - transcode on the fly

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• 34c2c10a - commenting the link ref

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• 321bb5d0 - changing the message in the graph

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• ed618d89 - testing

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