Update set-theory.html.markdown

This pull request attempts to fix major inconsistencies and some incorrect information. It also streamlines the article by rearranging and/or removing stubs of information.

- Replaced `v` and `^` set operators with cup and cap, respectively;
- Removed history section as the information was not necessarily accurate;
- Expanded symbols section by splitting it into operators and qualifiers;
- Renamed "builtin sets" to "canonical sets";
- Replaced Latin alphabet symbols for canonical sets with their Blackboard bold variants (see: https://en.wikipedia.org/wiki/Blackboard_bold);
- Rewrote set relations section to streamline information;
- Rewrote examples in set operations section to make meaning more clear;
- Removed generalized operations section, as the original author was clearly trying to typeset unions and intersections of collections of sets, but the notation is impossible to replicate in markdown and only makes the section more unclear.

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