Update from Bootstrap 3 to 5

[sebastiancarlos]

Very similar to the update done to taskwarrior.org: https://github.com/GothenburgBitFactory/tw.org/pull/930

Screenshots

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Notes:

• Code blocks were kept with a dark background. This is because we need them to have contrast with white text, which appears sometimes when displaying chart examples.
• But code blocks were kept with a light background in the rendered man pages.

[mattsmida]

Thanks Sebastian! Great to see the polish here on the timewarrior site in addition to your work on the taskwarrior site.

I'm merging these changes now, and I'm going to add two small things that I noticed during my review in another PR:

1. tw.org does not include a bootstrap-theme CSS file like ti.net does, leading to some inconsistencies between the two sites. So I'm going to remove that CSS file for ti.net.
2. The footer for ti.net appears smaller than that of tw.org. I'm going to update the CSS to get them to match.