Remove anchorjs in toc filter

[github-actions]

This PR Closes #638 and replaces acnhorjs in the TOC filter with an accessible vanila css/html solution.

Anchors are implemented by making the headers links themselves, rather than a separate link for the following reasons:

• it's a bigger tap area for mobile
• it avoids those Google SERP, RSS and Reader Mode issues
• it means you don't need to worry about the label text
• it allows you to hide the anchor icon (inserted with a ::before { content: icon }) on mobile or only when focus/hovered or remove it completely (though there are discoverability issues with some of those options).
• Its accessible with screenreaders. Voice Over reads it out as "This is a heading. Heading Level 2. Visited. In page link."

Reference implementation: https://codepen.io/johanjanssens/pen/PoWObpL

• MDN uses this approach (without an anchor icon): https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a#attributes

• Web Almanac (but with an anchor icon on desktop on hover/focus, but no anchor on mobile where there's less space): https://almanac.httparchive.org/en/2020/accessibility#ease-of-reading

Related discussion: valeriangalliat/markdown-it-anchor#82

Config options

The TOC filter offers the following config options:

• anchor: To turn the anchor link on or off, default `true``
• icon: To set a different icon for; default # (*)
• max_level: The max heading level to use for the table and for the anchors, default :6
• min_level: The min heading level to use for the table and for the anchors, default :2

(*) Note that the icon accepts both a single character or anything else supported by the css contentproperty. For more info see: https://developer.mozilla.org/en-US/docs/Web/CSS/content