docs: 0.19 changelog

[Jorropo]

This is mostly the only "big" user facing changes except resource manager, the rest is background work.

[BigLep]

@Jorropo : isn't this already covered in https://github.com/ipfs/kubo/blob/master/docs/changelogs/v0.18.md#new-default-pubsubseenmessagesstrategy ?

If so, then that means our only thing specific in Kubo was resource manager and everything else was in the go-libipfs layer? A couple of things about this:

1. Are we sure? There's nothing that percolated to the HTTP Gateway that Kubo exposes? (@hacdias or @lidel to confirm)
2. This is upping the importance of making sure our changelogs in go-libipfs are better so someone can go and learn about the improvements there.

[lidel]

Added Gateway section with IPIP-351 changes – fb663c6.

We also did ipfs/boxo#155, ipfs/boxo#150 and ipfs/boxo#187 but I feel these are low-level changes that end user is not interested about. Lmk if we should add a sentence about these.

[BigLep]

Thanks for the additions @lidel . They look good to me.

@Jorropo : I think you an merge once you've confirmed whether we aren't being redundant on "PubSub message caching improvements".

Updates also slipped in for #9708

[BigLep]

@Jorropo: can you please get this resolved and merged? Please confirm whether the pubsub comments are needed per #9707 (review)

[galargh]

I'm merging this as-is. Please feel free to update the changelog later.