Developer onboarding

For developer onboarding, there is development.md. I find it helpful, but not perfect. There are several things that can be improved.

## Link to development.md from README.md

If it was linked from README.md, it would save me some time. I wouldn't try docker-compose, which seems to be useful for production deployment rather than for development.

## Prerequisites

* JDK (not sure about versions that are OK; I've used JDK 17)
* SBT (latest version of sbt-launcher is OK, because it will download the required version)
* Elm compiler (I've used the latest version)
* Tests: tesseract unoconv wkhtmltopdf tesseract-langpack-deu

## Joex mentions

A link to https://docspell.org/docs/joex/intro/ would be useful.

BTW, the documentation doesn't show well on computer with a narrow window like `window.document.body.clientWidth == 701`. In Firefox@Fedora, it shows just the menu, without an ability to scroll it. When I make the window absurdly large (almost 3000px height, which is far larger than my screen), I can see the content under the menu. But maybe I should open a separate issue for this.

## Developing Frontend

### Starting restserver/joex

This section suggests to call `sbt "restserver/reStart"`. It doesn't work, because it launches the server in background and returns control back to SBT, which quits. I need to run `sbt` and then `restserver/reStart`.

### Modified frontend sometimes isn't reflected in browser

When I modify the frontend, the changes are reflected only when I use Ctrl+F5 (in Firefox). When I reload with F5, I get the old version, probably because the data is received from service worker (as shown in developer console). This happens even though I use `DOCSPELL_ENV=dev`.