Index update

[mahesh-panchal]

I made some changes to the front page.

• Remove git prerequisite: Git is not used in the lesson.
• Reduced jargon: I removed some jargon like deploy complex parallel and reactive pipelines.
• Spelling, punctuation, and grammar fixes.
• I reordered the lesson objectives as it makes more sense to write a workflow first before the amending configuration.
• I added an explanation of what Nextflow is.
• I shorted some descriptions to ease reading.