Add experimentalFetchPolyfill docs

[bahmutov]

updated places where we talk about network stubbing and warn about window.fetch

Used Markdown URL in the partial, because using {% URL ... %} was throwing an error

TypeError: Cannot read property 'data' of undefined
    at Object.getUrlProps (/Users/gleb/git/cypress-documentation/lib/tags/url.js:6:29)
    at /Users/gleb/git/cypress-documentation/lib/tags/url.js:88:24
    at tryCatcher (/Users/gleb/git/cypress-documentation/node_modules/bluebird/js/release/util.js:16:23)

for cypress-io/cypress#7597

[jennifer-shehane]

yah url tags don't work in partials #2422