docs: add initial documentation (#842)

docs/content/1.guide/1.index.md

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+# Introduction
+
+## What is Elk?
+
+::alert{type=warning}
+🚧 This section is a work in progress. 🚧
+::
+
+## What is Mastodon?
+<!-- -> external links -->
+
+::alert{type=warning}
+🚧 This section is a work in progress. 🚧
+::
+
+## What is a Mastodon Client?
+
+<!-- -> other examples -->
+
+::alert{type=warning}
+🚧 This section is a work in progress. 🚧
+::
+
+## Sponsors
+
+We want to thank the generous sponsoring and help of:
+
+[![NuxtLabs](/docs/images/nuxtlabs.svg)](https://nuxtlabs.com/)
+[![StackBlitz](/docs/images/stackblitz.svg)](https://stackblitz.com/)
+
+And all the companies and individuals sponsoring Elk Team members.
+
+[Find out more about sponsoring Elk](/guide/sponsoring).

docs/content/1.guide/2.features.md

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+# Features
+
+::alert{type=warning}
+🚧 This section is a work in progress. 🚧
+::
+
+<!-- once the UI is more stable, we can add screenshots -->
+
+<!-- - markdown support
+- code blocks
+- reordering and connecting posts in timelines
+- multi account
+- GitHub HTML cards
+- and so on... -->

docs/content/1.guide/3.contributing.md

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+# Contributing
+
+We're really excited that you're interested in contributing to Elk! Before submitting your contribution, please read through the following guide.
+
+## Online
+
+You can use [StackBlitz CodeFlow](https://stackblitz.com/codeflow) to fix bugs or implement features. You'll also see a CodeFlow button on PRs to review them without a local setup. Once the elk repo has been cloned in CodeFlow, the dev server will start automatically and print the URL to open the App. You should receive a prompt in the bottom-right suggesting to open it in the Editor or in another Tab. To learn more, check out the [CodeFlow docs](https://developer.stackblitz.com/codeflow/what-is-codeflow). 
+
+[![Open in Codeflow](https://developer.stackblitz.com/img/open_in_codeflow.svg)](https://pr.new/elk-zone/elk)
+
+## Local Setup
+
+Clone the repository and run on the root folder:
+
+```bash
+pnpm i
+pnpm run dev
+```
+
+`Warning`: you will need `corepack` enabled, check out the [Elk Contributing Guide](./CONTRIBUTING.md) for a detailed guide on how to set up the project locally.
+
+We recommend installing [ni](https://github.com/antfu/ni#ni), that will use the right package manager in each of your projects. If `ni` is installed, you can instead run:
+
+```bash
+ni
+nr dev
+```
+
+## Testing
+
+Elk uses [Vitest](https://vitest.dev). You can run the test suite with:
+
+```bash
+nr test
+```
+
+# Stack
+
+- [Vite](https://vitejs.dev/) - Next Generation Frontend Tooling
+- [Nuxt](https://nuxt.com/) - The Intuitive Web Framework
+- [Vue](https://vuejs.org/) - The Progressive JavaScript Framework
+- [VueUse](https://vueuse.org/) - Collection of Vue Composition Utilities
+- [Pinia](https://pinia.vuejs.org/) - The Vue Store that you will enjoy using
+- [Vue Macros](https://vue-macros.sxzz.moe/) - More macros and syntax sugar for Vue
+- [UnoCSS](https://uno.antfu.me/) - The instant on-demand atomic CSS engine
+- [Iconify](https://github.com/iconify/icon-sets#iconify-icon-sets-in-json-format) - Iconify icon sets in JSON format
+- [Masto.js](https://neet.github.io/masto.js) - Mastodon API client in TypeScript
+- [shiki](https://shiki.matsu.io/) - A beautiful Syntax Highlighter
+- [vite-plugin-pwa](https://github.com/vite-pwa/vite-plugin-pwa) - Prompt for update and push notifications

docs/content/1.guide/4.sponsoring.md

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+# Sponsoring
+
+If you're enjoying the app, consider sponsoring our team:
+
+- [Anthony Fu](https://github.com/sponsors/antfu)
+- [Daniel Roe](https://github.com/sponsors/danielroe)
+- [三咲智子 Kevin Deng](https://github.com/sponsors/sxzz)
+- [Patak](https://github.com/sponsors/patak-dev)
+
+We would also appreciate sponsoring other contributors to the Elk project. If someone helps you solve an issue or implement a feature you wanted, supporting them would help make this project and OS more sustainable.