docs: explain the folder structure of src & tests

These will help onboarding new contributors, I hope.

Signed-off-by: André Jaenisch <andre.jaenisch@posteo.de>

src/README.md

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+<!--
+SPDX-FileCopyrightText: 2023 Pere Lev
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
+# Source
+
+These files here provide the server and client code of Anvil.
+We try to use TypeScript for an additional level of understandability.
+However, components cannot use TypeScript and have to be written in JavaScript
+in order to work with the component library. Otherwise their annotation cannot
+be parsed.
+
+- `lib` - Files that can be referenced by [`$lib`](https://kit.svelte.dev/docs/modules#$lib).
+  We use them for Svelte components,
+  [internationalisation](https://www.npmjs.com/package/svelte-i18n) and
+  [server-only modules](https://kit.svelte.dev/docs/modules#$lib-$lib-server).
+  This approach allows us to also surface a component library.
+- `routes` - [SvelteKit routing](https://kit.svelte.dev/docs/routing)
+- `README.md` - This document.
+- `app.d.ts` - [SvelteKit App type definitions](https://kit.svelte.dev/docs/types#app)
+- `app.html` - [SvelteKit page template](https://kit.svelte.dev/docs/project-structure)
+- `app.postcss` - Global styles.
+- `hooks.server.ts` - [SvelteKit server hooks](https://kit.svelte.dev/docs/hooks#server-hooks)
+- `theme.postcss` - Theming.

tests/README.md

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+<!--
+SPDX-FileCopyrightText: 2023 Pere Lev
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
+# tests
+
+These are the tests for our application.
+Right now there are mainly component tests because those are under development.
+
+- `components` - Components tests matching the folder structure of `src`.
+- `README.md` - This document.
+- `test.ts` - An example end-to-end test.