Enhancement: provide API overview in docs

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## Feature request

First of all, thank you for creating Sharp! It's proven very useful for a number of my projects and is what I reach to first when needing any sort of image transformations or conversion. 😄

I'd be open to writing a PR for the following if it's something that is wanted on the Sharp docs.

### What are you trying to achieve?

- Currently to start using Sharp, you need to visit the Installation page and install, and then scroll through the Constructor page to find out how to construct Sharp (and import sharp - is it a default import? Named import? Docs don't really say), and then search through the docs on Resizing Images or other API doc pages for common use-cases.
- It would be nice to have a "quickstart" or "getting started" guide on the docs, similar to many other open-source documentation sites.
- A getting started guide is useful to help the user in getting Sharp installed and used as fast as possible.
- It doesn't need to be comprehensive or very long, that's what the rest of the docs are for, but a simple "visit the Installation page to install, then import sharp like this, construct sharp like this, here are some common use-cases, see docs for the rest" might be useful.

### When you searched for similar feature requests, what did you find that might be related?

- No related issues or feature requests.

### What would you expect the API to look like?

- N/A, docs change only.

### What alternatives have you considered?

- A section on importing Sharp within the existing Installation section. This would allow easier onboarding but would still require the user to look through the rest of the docs for common use-cases.

Start by reading the existing Installation, Constructor, and Resizing Images documentation to understand the current onboarding path and import guidance. Done means adding a concise getting-started or quickstart guide that covers installation, importing Sharp, constructing it, and a few common use cases without duplicating the full API documentation.

Written by the indexing model from the issue text.