Update quickstart with Latest Tutor Commands

[feanil]

Section: Mounting edx-platform

Noted areas of concern include:

1. The provided command tutor dev run --mount=./edx-platform lms bash to mount to a local edX checkout is deprected.
2. Bashing into the LMS container to set up is now redundant and performed by Tutor automatically on launch.
3. The steps provided can be significantly reduced to the following flow: fork, mount, and launch as specified here.

Source of information:

1. Official Tutor CHANGELOG
2. Official Tutor Documentation

Additional information:
This section required updates beyond minor "suggested edits". The updates can be observed in this feature branch.
If applicable, I can create a new pull request.

Your advice on merging the necessary update to the developer Quickstart is appreciated.

Originally posted by @apexDev37 in #368 (comment)

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@apexDev37 thanks for the info, do you want to take a stab at making the update?