Add 4.5.21 release notes

[kriszyp]

There aren't any indexes I need to update? And I am not sure what sidebar_position is?

[heskew]

nothing else to update. the sidebar and search index will be updated automatically. sidebar position is the current way the sidebar is sorted in descending version order, decrement from 2.5.20 and it'll be in the correct order. long term goal is to not need to manage that.

[heskew]

also not documented anywhere, the sidebar pos is decremented 100 for each minor version, leaving room for 99 patch revs below

[heskew]

the release notes index hasn't been updated so that's something that can be done now, or in a followup. would be nice to generate this too.

https://github.com/HarperDB/documentation/blob/main/docs/technical-details/release-notes/index.md

https://docs.harperdb.io/docs/technical-details/release-notes/

[Ethan-Arrowood]

Pushed a fixup for the sidebar positioning and also copied over some missing release notes to docs/

Sorry this process is a little annoying right now. I promise we'll improve it soon!

[kriszyp]

Thank you!!!!

[Ethan-Arrowood]

@heskew since I committed to this, can you do a quick review for Kris and I? Thanks!