[functorch] Add more details to the functorch install page

[zou3519]

Stack from ghstack:

• -> [functorch] Add more details to the functorch install page #86823
• Add vmap support for slogdet; fix regression from functorch 0.2.1 #86815

Added some details about:

• pip uninstall functorch being helpful if there are problems
• pip install functorch still working for BC reasons.

Test Plan:

• wait for docs preview

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/86823

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As of commit d407bd1:
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[zou3519]

Preview page over at https://docs-preview.pytorch.org/86823/functorchdocs/install.html

[zou3519]

@pytorchbot merge -f "docs-only change"

[pytorchmergebot]

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[github-actions]

Hey @zou3519.
You've committed this PR, but it does not have both a 'release notes: ...' and 'topics: ...' label. Please add one of each to the PR. The 'release notes: ...' label should represent the part of PyTorch that this PR changes (fx, autograd, distributed, etc) and the 'topics: ...' label should represent the kind of PR it is (not user facing, new feature, bug fix, perf improvement, etc). The list of valid labels can be found here for the 'release notes: ...' and here for the 'topics: ...'.
For changes that are 'topic: not user facing' there is no need for a release notes label.