store/metadata: Update docs with recent changes

[kakkoyun]

https://cloud-native.slack.com/archives/C01BB0M6K1U/p1644325866492779

[metalmatze]

Copied from Slack:

Hello, I’m messing about with Parca today on my 20% time, and I was just wondering… The docs say

for now, we’re using a SQLite in-memory by default
https://www.parca.dev/docs/storage

but it looks like you’re using Badger by default, which to my admittedly limited understanding is a KV store
https://github.com/parca-dev/parca/blob/v0.7.1/pkg/parca/parca.go#L73

[Dean-Coakley]

Looks like this can be closed as it was resolved in: #128 ?

[brancz]

We actually just changed this again which made the updated docs stale again. The organization of keys is thoroughly documented here though: https://github.com/parca-dev/parca/blob/main/pkg/metastore/kv.go