feat(js): Add micro-frontend docs

[AbhiPrasad]

Closes getsentry/sentry-javascript#7622

Adds preliminary docs for using the multiplexed transport introduced with getsentry/sentry-javascript#7926 for the micro-frontends use case.

Adds a warning about async context support meaning that we cannot isolate breadcrumbs/spans.

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Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 2, 2023 0:08am

[shanamatthews]

language tweaks, style tweaks, fenced code block language standardization

[kenrick95]

Random question but might be relevant to document: which ones are called first: the "matcher" ("dsnFromFeature") or the init's "beforeSend"?

As an example, if I enhance my event tags at init's beforeSend (e.g. I add "event.tags.feature = cart"), will the enhanced event tags be available at this matcher (i.e. at dsnFromFeature when getEvent() is called)?

Context: before this multiplexed transport feature is available, one workaround to send different event to different Sentry project is via beforeSend and initialize a BrowserClient there (https://github.com/sentry-demos/sentry-micro-frontend/blob/5996c4b528f58696b729db9d41f4962a83c53c77/methods/simple-remote.js#LL42C28-L42C28) So when implementing this mechanism at my company, my implementation actually adds more tags there than just finding out relevant DSN to send to (e.g. I added the micro app's release info). If I could access the tags added in "beforeSend" when the matcher is called, I could reduce some code duplication. However if that's not possible, it's also ok too for me.

Thanks~

[AbhiPrasad]

The init's beforeSend is called before the matcher's dsnFromFeature. Will call this out in the docs - thanks for updating it!

[AbhiPrasad]

Thanks for the review @shanamatthews!