feat: development container with Codespaces compatibility

[almereyda]

• adds pagefind.yml to automate pagefind runs with less command line parameters
• adds .devcontainer.json

It can be used with

• Microsoft GitHub Codespaces
• Microsoft Visual Studio Code
• the Development Containers CLI

• updates package.json to contain dedicated build and develop scripts supersedes osuny.js
• updates README.md to mention the development container

---

• feat: add development container degrowth/idn-website#14
• fix: devcontainer compatibility with Codespaces degrowth/idn-website#16

---

To test this during review before the merge, do this:

1. Navigate to https://github.com/codespaces/new
2. Choose the repository degrowth/idn-website and click Create codespace
3. Run the yarn codespaces command, as instructed by the Readme

Alternatively you can open a/the codespace from within the repository:

1. Navigate to https://github.com/degrowth/idn-website
2. Click the green <> Code button
3. Switch to the Codespaces tab
4. Click the button with the ➕ icon.
   A new tab with Visual Studio Code opens. Wait for everything to complete.
5. Open a new terminal by clicking on the ➕ button in the lower right
6. Run the yarn codespaces command, as instructed by the Readme

closes #8