docs: coding style

docs/coding-style.md

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+# TypeScript
+
+- avoid `any`; get rid of any existing `any` whenever you can so that we can enable `"strict": true` later on in `tsconfig.json`
+- define custom types for common data structures
+  - don't worry about `interface` vs `type`, [both are fine](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#differences-between-type-aliases-and-interfaces)
+
+## Typescript and React/Next
+
+- use `React.FC<Props>` type for React components, e.g. `const MyComponent: React.FC<Props> = ({ ... }) => { ... };`
+- use `NextPage<Props>` for typing stuff in `src/pages/`
+- use generic versions of `GetServerSideProps<Props>` and `GetStaticProps<Props>`
+
+# React
+
+- create one file per one component (tiny helper components in the same file are fine)
+- if
+- name file identically to the component it describes (e.g. `const DisplayForecasts: React.FC<Props> = ...` in `DisplayForecasts.ts`)
+- use named export instead of default export for all React components
+  - it's better for refactoring
+  - and it plays well with `React.FC` typing
+
+# Styles
+
+- use [Tailwind](https://tailwindcss.com/)
+- avoid positioning styles in components, position elements from the outside (e.g. with [space-\*](https://tailwindcss.com/docs/space) or grid/flexbox)
+
+# General notes
+
+- use `const` instead of `let` whenever possible
+- set up [prettier](https://prettier.io/) to format code on save