When I gave #8097 a quick check locally, my local markdown updates weren't going through and I noticed it was fetching everything from the server.

Turns out this weird behavior https://github.com/microsoft/pxt/pull/7989/files#r598994321 of updating as a side effect was the thing preventing it from serving docs from the server instead of the local versions when doing a pxt serve, as setting that to true makes us skip the local request path here:

Fix that so we don't go to makecode.com/api/* for markdown on pxt serve. (the changes are just guarding with pxt.BrowserUtils.isLocalHostDev() in a few places where we don't want to default to fetching from server -- e.g. we still want to get updates when if we're doing incontext translations or if we pass livelang=de)