docs: add frontend docs (AI Assisted)

[tohega]

Description

This was co-created with the onboarding Claude Skill from inovex and me, due to my timely departure into another project I did not thoroughly review the output more than reading it once and comparing it to my current knowledge of the repo.

However I feel like this serves as a good basis for future onboardees in the frontend to get up to speed quickly.

Read the docs here:

From the docs/ directory:

yarn dev
That serves on http://localhost:4321/ with hot reload. yarn start is an alias for the same thing.

yarn build
and then
yarn preview

Serves the built dist/output (search indexing will work only in build appearently)

Changelog

Checklist

• I have performed a self-review of my own code
• I have written and understand every part of this contribution myself - if AI tools were used, I have thoroughly reviewed and verified all changes
• I have commented my code, particularly in hard-to-understand areas
• The light- and dark-theme are both supported and tested
• The design was implemented and is responsive for all devices and screen sizes
• The application was tested in the most commonly used browsers (e.g. Chrome, Firefox, Safari)

(Optional) Visual Changes

[github-actions]

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Deployed Images

• ghcr.io/inovex/scrumlr.io/scrumlr-frontend:sha-fe5fca5

• ghcr.io/inovex/scrumlr.io/scrumlr-server:sha-fe5fca5