docs: NO-TICKET: Add dependencies to getting started. (#4014)

* Add dependencies to getting started.

This sort of requirement is often missed since developers usually already have the dependencies installed for the product they are developing... obviously :)

This PR adds a new first step to have the dependencies installed before installing label-studio from `pip`. It also adjusts the numbered steps afterwards accordingly.

Tip: Depending on your markdown renderer you can use `1.`  for all steps and it will auto-increment in the render, making changes to numbered steps like this simpler. It also standardizes the indent to nest blocks under the `ol` they generate, which is another possible formatting improvement (again, depending on what's rendering the markdown).

* docs: LSDV-4880: Small docs fixes with misspells: update get_started.md

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Co-authored-by: Max Tkachenko <makseq@gmail.com>