Add metatags to show preview content when sharing website links

### What is the problem or limitation you are having?

When sharing pages on the BeeWare website, there is no "social sharing" content in the page metadata, so social media sites show a "default" webpage preview

### Describe the solution you'd like

When sharing a page from the BeeWare website, a social-enhanced preview page should be displayed.

Pages like blog posts and event notifications should have specialised previews that highlight the page content.

### Describe alternatives you've considered

-

### Additional context

It's possible MkDocs has this feature available as a plugin.

As an example of the sort of thing we're talking about: here's a [social preview card for a SciPyLA](https://2019.scipyla.org/ponentes/travis-oliphant/)

```html

```

Which renders on slack and similar as: