docs: add local development quickstart to README (fixes #1926)

[aryanrahar]

Adds a concise “Local development quickstart” so new contributors can build and run tests immediately without leaving the page.
Uses build-infer.sh + make devsetup.
Documents fast OCaml inner loops (-C infer/src check and byte).
Mentions BUILD_MODE=dev-noerror for temporary iteration.
Shows infer run -- make|mvn for analyzing real builds.

Mirrors and links to INSTALL.md and website docs to avoid divergence.

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[aryanrahar]

Can take a look? cc @freddyboulton