Finalize naming of contributes.css / contributes.html and update JSON schema / docs

[octref]

• HTML: Add contributes.html.customTags and contributes.html.customAttributes #64022
• CSS: Allow contributes.css.customData #66414

Once both are done, we should think of consistent names and

• Add them to the JSON schema for package.json.
• Update docs at https://code.visualstudio.com/api/references/contribution-points or in a new topic similar to https://code.visualstudio.com/api/extension-guides/markdown-extension.

[aeschli]

Here are some alternatives to 'custom data':

vocabulary
syntax identifiers

[octref]

Schemas and docs are published https://github.com/Microsoft/vscode-html-languageservice/tree/master/docs and https://github.com/Microsoft/vscode-css-languageservice/tree/master/docs.