Docs: Capitalisation of title line changed to lower case

[HolgerSpernau]

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[jorisvandenbossche]

There is already somebody working on this: #26933
So best to first check how that overlaps (don't have time to check now, just quickly wanted to note that)

[gfyoung]

Closing in favor of #26950.

@HolgerSpernau : Thanks for your interest in contributing! Although this time you stumbled upon on issue that was already being worked on, there are plenty of other open issues where your help would be welcomed and appreciated!

[HolgerSpernau]

@gfyoung : Thanks. I think the work I have done can still be used. I understood in #26933 that it is not only related to the first word in the header, but also the second or third words. That's what I concentrated on. Have a look again on the changed. Thanks.

[gfyoung]

@HolgerSpernau : That PR is for all words. In addition, we're going with as much title case (in general) as possible, whereas your PR goes for lowercase. Thanks for checking though!