[2/x] docs update: separate quick start

[vkuzo]

1. move eager mode quant quick start into index.rst (no content change)
2. move pt2e quick start into pt2e_quantization (no content change)
3. delete top-level quick start section

all the actual sections will change in future PRs, this PR is just
moving lines of code around

Summary:

Test Plan:

[vkuzo]

Stack from ghstack (oldest at bottom):

• [5/x] docs update: initial clean up of quantization API reference section #3763
• [4/x] docs update: move workflow overview section from README to docs #3762
• [3/x] docs update: make docs landing page be like the main README #3761
• -> [2/x] docs update: separate quick start #3760

[pytorch-bot]

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