Add rule copy link buttons

[SmilyOrg]

Rule headings could contain a variety of small icons at the end of them for quick copy-pasting into documents, chats, and comments.

I've thought about two specifically for now - the second one of which would be most handy for PRs and GitHub issues.

1. 🔗 - Regular link

This just copies the URL to clipboard and nothing else. One less action compared to now :)

2. - Markdown link

Copies a Markdown-formatted link to clipboard so you can very easily reference it in e.g. a GitHub issue. This would make it easy to produce meaningful links instead of raw URLs or manual editing.

Some format ideas:

1. [MUST use standard formats for date and time properties [169]](https://opensource.zalando.com/restful-api-guidelines/#169)
2. [MUST use standard formats for date and time properties](https://opensource.zalando.com/restful-api-guidelines/#169)
3. [REST [169] MUST use standard formats for date and time properties](https://opensource.zalando.com/restful-api-guidelines/#169)
4. [RESTful #169](https://opensource.zalando.com/restful-api-guidelines/#169)
5. [REST #169](https://opensource.zalando.com/restful-api-guidelines/#169)

Markdown icon source: https://commons.wikimedia.org/wiki/File:Markdown-mark.svg