docs: add warning about Chrome Private Network Access restrictions

[ivan-aksamentov]

Chromium-based browsers block requests from HTTPS sites to localhost due to Private Network Access security restrictions. This affects users trying to load local files via URL parameters. Documentation now explains the restriction and provides workarounds.

Example error:

Access to XMLHttpRequest at 'http://localhost:3001/sequences.fasta' 
from origin 'https://clades.nextstrain.org' has been blocked by 
CORS policy: Permission was denied for this request to access the 
`unknown` address space.

localhost:3001/sequences.fasta:1  Failed to load resource: 
net::ERR_FAILED

Sometimes shows user a dialog:

For dev purposes, disable this flag, in a separate dev instance of chromium (but never in your main browser!):

chrome://flags/#local-network-access-check

or run chrome(-ium) with:

--disable-features=LocalNetworkAccessChecks

• Document PNA restriction affecting localhost URLs
• Add workarounds: drag-and-drop, CLI, public HTTPS hosting, chrome://flags

[github-actions]

Preview: https://nextstrain--nextclade--pr-1753.previews.neherlab.click

(ci)