[docs] Describe '--cache' option

[miherlosev]

Need to do

• Describe the --cache option.
• Add paragraph to the Decrease test execution time topic.

Draft "--cache" option description:
The --cache option enables the caching of the website resources (styles, script, images, etc.) on the TestCafe's proxy side.

TestCafe emulates the native browser behavior and cache only these files that the browser will cache without TestCafe.
TestCafe does not cache the HTML pages and some other resources because it may break the Roles feature.
The use of this feature can be useful if your application loads a lot of resources (scripts, styles, fonts, etc.) on the first visit and cache them. Note, that this feature is experimental.

The '--cache' option is available in 3 interfaces: command-line, programmatic, and configuration file.
Command-line

testcafe chrome tests --cache

Programmatic

runner.run({ cache: true });

Configuration file

{
 "cache": true
}

[DIRECTcut]

fixed in #5969