Explicitly set the id attribute of headings in the Markdown documents to keep them consistent across translations.

[ilyaspiridonov]

In the English documentation, id attributes of headings that are cross-referenced in the document, must be set explicitly in Markdown, to make sure those ids are consistent across translated documents and anchor links are working correctly on non-English pages.
Example:

## Web Component {: #web-component }

Support for explicit id attributes of headings is added in this PR:
#162

[ilyaspiridonov]

@patrickkettner FYI