Theme: Improve rendered artifact filesize and loading time

While the current theme code is fairly decent (average of 400kB per page on 5K pages and almost 1K processed images, total of 200MB raw *non-brotlied*), there are many improvements and optimizations possible, specially important as the website may be viewed many thousand times per day around the world:

Template:
- Optimize images and assets filesize, at the expense of build time
- Reduce total page count, taxonomy terms and pagers.
- Improve resources minification, loading and synchronization: images, JS, CSS, etc.
- Improve HTML and CSS structure with performance in mind

Config:
- Optimize settings and config