Content pages for reusable content only

[ScriptRaccoon]

Proofs of properties of a given category (or functor, etc.) can be given directly on the category page and therefore written in the category file. Since proof popups can be expanded (#316), even long proofs remain easy to read. Content pages should only be used for reusable or expository content.

Consequently, three content pages have been removed. The proofs that Meas is not regular, that Ab_fg has $\aleph_1$-cofiltered limits, that BN and BOn have $\aleph_1$-filtered colimits, and that BN is $\aleph_1$-accessible have been moved directly to the respective category pages.

Advantages:

• less indirection
• the proofs for a given category can be found in one file and therefore also compared with each other
• clearer separation of concerns
• easy rule (before, it was unclear whether a proof should go on a content page or in the category file)
• lots of proofs were following this rule already (such as the long proof that Unif is co-Malcev)

The proof length script (#221 + #238) has also been removed, as it is no longer needed. It was not running in CI anyway. Long proofs are OK.