Update links in installation guide (#4899)

docs/conf.py

@@ -283,6 +283,7 @@ coverage_ignore_pyobjects = [
 intersphinx_mapping = {
     'attrs': ('https://www.attrs.org/en/stable/', None),
     'coverage': ('https://coverage.readthedocs.io/en/stable', None),
+    'cryptography' : ('https://cryptography.io/en/latest/', None),
     'cssselect': ('https://cssselect.readthedocs.io/en/latest', None),
     'itemloaders': ('https://itemloaders.readthedocs.io/en/latest/', None),
     'pytest': ('https://docs.pytest.org/en/latest', None),

docs/intro/install.rst

@@ -69,10 +69,9 @@ In case of any trouble related to these dependencies,
 please refer to their respective installation instructions:
 
 * `lxml installation`_
-* `cryptography installation`_
+* :doc:`cryptography installation <cryptography:installation>`
 
 .. _lxml installation: https://lxml.de/installation.html
-.. _cryptography installation: https://cryptography.io/en/latest/installation/
 
 
 .. _intro-using-virtualenv:
@@ -265,7 +264,6 @@ For details, see `Issue #2473 <https://github.com/scrapy/scrapy/issues/2473>`_.
 .. _cryptography: https://cryptography.io/en/latest/
 .. _pyOpenSSL: https://pypi.org/project/pyOpenSSL/
 .. _setuptools: https://pypi.python.org/pypi/setuptools
-.. _AUR Scrapy package: https://aur.archlinux.org/packages/scrapy/
 .. _homebrew: https://brew.sh/
 .. _zsh: https://www.zsh.org/
 .. _Scrapinghub: https://scrapinghub.com