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

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:
[https://github.com/ampproject/bentojs.dev/pull/162](https://github.com/ampproject/bentojs.dev/pull/162)