extractred --> extracted

Add Python3 tools description.

Corrected minor grammatical issues and increased clarity of instructions.

I changed URLs to :class:`~scrapy.http.Request` in start_urls explanation of the default spider

Rectified grammatical errors

Add mention to dont_merge_cookies in CookiesMiddlewares docs (#2999)

[CI skip]

Thanks @eliasdorneles!

* suggest Stack Overflow for Scrapy usage questions;
* encourage users to submit test-only pull requests with reproducable examples;
* encourage users to pick up stalled pull requests;
* we don't use AUTHORS file as a main acknowledgement source;
* suggest using Sphinx autodocs extension

Fixed spelling mistake from "will be pass" to "will be passed"