Improve docs

[FloEdelmann]

1. Remove outline: deep frontmatter which is now unnecessary

• Follows Show nested outline in docs #2481.

2. Bump eslint-plugin-jsonc to latest version

• I somehow missed this in Update development dependencies #2478, that's probably causing errors like this.

3. Also expose SourceCode in eslint shim

Fixes this issue in https://eslint.vuejs.org/rules/max-lines-per-block.html:

because SourceCode is needed in

eslint-plugin-vue/lib/rules/max-lines-per-block.js

Line 7 in c64bf94

const { SourceCode } = require('eslint')

[ota-meshi]

Thank you!