docs: various small fixes

[Staudey]

Description

This PR makes a number of small improvements/fixes all over the docs:

• repo-wide: Use bash syntax-highlighting where appropriate.

• repo-wide: update/fix links

• local-repository: common no longer has to be updated separately; link to update-dev-environment instead

• packaging-changes: Remove duplicated "October" header, fix link to Taskfile.yml

• prepare-for-packaging: use bash highlighter for code blocks instead of zsh/fish

  • Prism doesn't have highlighters for fish and zsh (https://prismjs.com/#supported-languages)

• user/intro: fix capitalization and typo

• package-management: remove sudo where not necessary

• secure-boot: tiny clarification regarding clr-boot-manager version

• quick-start/package-management: rename to sofware-center and remove eopkg.md, link to user/package-management section instead to reduce duplication

• desktops: fix and clean up GNOME instructions

Submitter Checklist

• [x] Squashed commits with git rebase -i (if needed)

[Staudey]

Converted to draft because I'll try to deduplicate some repo management pages too

[Staudey]

Okay, should be ready for review now