Use sentence casing in headers

A good case can be made for why sentence casing the headers is better. I'll copy some points I got from NameLessGo.

## Why use Title Case?
>
>* Let’s Make It Official. Looks serious, right? Exactly!
>
>* Grabs reader attention. Capitalized phrases stand out from the crowd, as [Pieter Heyman](https://uxdesign.cc/writing-styles-for-a-better-ui-and-ux-cdc35b457444) nicely put it.
>
>* Keeps a nice visual rhythm. Of course, as long as the sentences are short.
> * [Stands out](https://medium.com/@jsaito/making-a-case-for-letter-case-19d09f653c98). Title Case differentiates title from the body text.
>
> * Maintains symmetry. Yet, works only for short texts (ex. Buttons, Labels)
>
>
> ## Why use Sentence case?
>
> * Consistency. Easier to maintain across different platforms.
>
> * Clarity. Easy to navigate and skim, thank you, [John Saito](https://medium.com/@jsaito/making-a-case-for-letter-case-19d09f653c98). (ex. Home > About us)
>
> * People love it. Yes, we love having human-ish conversations with apps.
> [Casual](http://blogs.quovantis.com/why-letter-casing-is-important-to-consider-during-design-decisions/). Gives an impression that anyone can use the product.
>
> * Easier to localize. Much much easier and less stressful.

[Original source for the above](https://uxdesign.cc/title-case-vs-sentence-case-in-ux-writing-212087192261)
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I think the main reason this manual should use sentence casing in headers is because educational apps, including Anki, is perceived as dry and boring. Title casing and serious language just enforces that sternness.

Here's an example of how it might look like with sentence casing: https://developers.google.com/style/

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I did start this long ago: https://github.com/ankitects/anki/issues/5468 but never got around making the changes. This is a fresh discussion about sentence case vs title case, not the same as that one. I actually tried to start this on forums but I wasn't sure which category to use so I just started here.

The issue does not name files, tests, or entry points; begin by locating the manual headers and reviewing the linked Google style guide and cited casing discussion. Done would require an agreed scope and consistent sentence casing across the selected headers.

Written by the indexing model from the issue text.