DOC update "Contributing" docs

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

[MRG+1] Issues a warning when user puts a URL into allowed_domains (#2250)

[MRG+1] Scrapy Command: add --meta/-m to the "parse" command to pass additional meta data into the request

checking for subclass of URLWarning instead of checking error message text when URL in allowed_domains

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

[MRG+1] Added from_crawler to middleware docs

Separated import statements

Just separated the import statements. Tiny change - testing GitHub!

[MRG+1] Revise/modernize item exporter example in docs

[MRG+1] Use https link in default user agent

Fix PyPy build

They are provided by https://github.com/squeaky-pl/portable-pypy

Fix broken links and use https links wherever possible

Use https for external links wherever possible in docs

[MRG+1] Add option to disable automatic log handler install

[MRG+1] Changed unhelpful log message from core.downloader.handlers.http11

[MRG+1] Add m4v extension to IGNORED_EXTENSIONS in LinkExtractor.

[MRG+1] Bump Twisted requirement on Python 3 to 17.9.0 to catch many Python 3 fixes.

Link "Debugging in Python" article to its new location

Add note about request.meta['depth'] in DepthMiddleware

Update tutorial.rst startproject files

updated file structure to include middlewares.py