Update Pelican webassets plugin

[jondricek]

We've been using a Pelican plugin for webassets for about 5 years by vendoring it here: https://github.com/mitre-attack/attack-website/tree/master/plugins/assets

This has caused it to become stale, while Pelican itself has updated their plugin approach to favor namespace plugins instead of the legacy format (https://docs.getpelican.com/en/stable/plugins.html#where-to-find-plugins)

The latest webassets plugin can be found here: https://github.com/pelican-plugins/webassets, which can be installed with python -m pip install pelican-webassets and therefore will be much easier to maintain.

This is worth looking into for the sake of maintainability.