Misc Documentation changes

[hasezoey]

Summary

This PR does some miscellaneous documentation improvements / changes:

• remove file prior.pug, because it is not generated anywhere since at least mongoose 4.0
• remove file contributing.md (in /docs), because that file was not listed in any sidebar and didnt provide any helpful things
• change api.pug to be api.md, to lessen the amount of pug files (re Switch from Pug to server side rendered Vue for docs #10312)
• change to use fileName in api_split.pug
• convert some documentation scripts from var to const / let
• fix Query.prototype.toStringTag not having a ctx.constructor
• change Symbols to now be listed as [Symbol.function] instead of .function or .Symbol.function
• add extra handling for symbols to be listed without a extra dot (now listed as .prototype[Symbol.function] instead of .prototype.[Symbol.function])
• fix schematypes's isRequired being listed as validators in documentation and at runtime
• change search script to use history.pushState instead of always reloading
• change search script to also use navbar search and disable nav-bar search when being in /search.html (but encourage the use of the big input)
• update purecss from 1.0.0 to 1.0.1