Add experimental sessions docs

[ascorbic]

Description (required)

Adds docs for the experimental Sessions feature

Related issues & labels (optional)

withastro/roadmap#1055

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Tracked Files

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en/reference/experimental-flags/sessions.mdx	Localization added, will be marked as complete.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[sarah11918]

Left an editing pass! Mostly breaking the dense config info up into bite-sized pieces.

Also discovered:

• unstorage's docs aren't great at hinting at what people should use as a driver name in our config (I think I handled this OK)
• unstorage's docs don't seem to reveal driver options well? (e.g. we show an example configuring a consistency option that is not mentioned on their site, so I don't know how people are going to know what's available to them). I have glossed over this, and am fine with this especially as a first experimental release. In general, I'm not thrilled that we are relying on some magical undocumented options and telling people to go look at them in a place where they're not, but I can't really do much about this.
• the name unstorage appears to be lower-case, so this is intentional throughout.

[sarah11918]

updating branch to trigger another Netlify deploy and hope it "just works" this time

[sarah11918]

Just waiting to see whether we need to update our references to getting driver names from unstorage, but otherwise, this will be good to go!