Fix broken links in docs

[FloEdelmann]

Fixes #1802.

My process:

1. Search and replace all:

   - https://v3.vuejs.org/style-guide/#([\w-]+?)-(essential|strongly-recommended|recommended|use-with-caution)
   + https://vuejs.org/style-guide/rules-$2.html#$1

   Then manually go through all changed links and adjust them if necessary: e756226
2. Search and replace all:

   - https://v3.vuejs.org/guide/migration/([\w-]+).html
   + https://v3-migration.vuejs.org/breaking-changes/$1.html

   Then manually go through all changed links and adjust them if necessary: d195cab
3. Search and replace all:

   - https://vuejs.org/v2
   + https://v2.vuejs.org/v2

   Then manually go through all changed links and adjust them if necessary: 6a3070e
4. Manually go through all https://v3.vuejs.org/ links and adjust them: 143d896
5. Temporarily install https://github.com/tcort/markdown-link-check with npm i -D markdown-link-check
6. Run find -name "*.md" -not -path "./node_modules/*" -print0 | xargs -0 -n1 npx markdown-link-check --progress --quiet --alive 200 --retry and fix all failing links: bc5cc10