Add clear instructions on how to contribute to git-send-email.io

Update Gmail-specific instruction

It is not possible to use regular SMTP with login and password for
Google accounts without enabled two-factor authentication[1]. App
passwords page just says "The setting you are looking for is not
available for your account" is such case.

[1] https://support.google.com/accounts/answer/185833

Update support email address to sr.ht-discuss

docs: improve Protonmail instructions

Setting up git send-email with the Protonmail Bridge requires disabling
SSL verification with the Protonmail Bridge, so we add this
documentation to help out new users.

Update information about Protonmail

* index.html: mention Protonmail Bridge.

add omnios

Make the output of step 4.4 closer to what users actually see

Mentioning src/main.c with a diff that is different than what the user
will experience when following the tutorial caused my OCD alarm to go
off.

.builds: alpine 3.11

Set default text color to black.

“background-color” is set in several places without a corresponding
“color”.

Fix typo in email tutorial success message

Add whitespace to smtpserveroption for example consistency

Remove exclamation points from echo commands

The echo commands in step 3 may not work as expected for users who are
not aware of Bash history expansion.

Bash adds a dquote continuation line:
echo "I'm about to try git send-email!" >your-name

Omitting the bang works:
echo "I'm about to try git send-email" >your-name

Also works without double quotes:
echo I\'m about to try git send-email! >your-name
echo Im about to try git send-email! >your-name
echo 'Im about to try git send-email!' >your-name

Signed-off-by: Brendon Smith <br3ndonland@protonmail.com>

.builds: update to alpine 3.11

Check new_lines value to validate patch

Looks like lib2git includes the optional/conditional 'No newline at
end of file' line within the lines collection of the hunk, which could
throw off the new line count.  So rather than that, try checking the
new_lines value to make sure that is 1.

One can see how this scenario is causing the test to fail with the
following patches

Fail:
https://lists.sr.ht/~sircmpwn/email-test-drive/patches/9184

Success:
https://lists.sr.ht/~sircmpwn/email-test-drive/patches/9187

Allow Unicode curly quote in stage 1 message

For those of us who created their file in a text editor which magically
uses Unicode quotes in prose.

Add example of how to cache the smtp password

Pin builds to Alpine 3.10

Remark on possibly missing SSL modules on macos.

As far as I can tell, the Developer Tools-bundled git/perl generally
is missing these modules. I believe that command worked for me, but
it would be good to verify on a fresh system.

Document smtpserveroption in Configuration

Signed-off-by: Matt Critchlow <matt@critchlow.xyz>

Fix typo: comprhensive -> comprehensive

Add link to git-rebase.io