git-terminal.md: add commands to update a fork and change email

[marchersimon]

Many new contributors don't know how to update their forks and change their commit email if they have trouble with CLAssistant. git-terminal.md seems like the perfect place to document that. Feel free to reword anything.

[navarroaxel]

You can add the new Fetch upstream button https://twitter.com/github/status/1390382527588798477

[CleanMachine1]

I, to be honest, don't know how to fetch upstream between forks, I have just been using the really useful new fetch upstream button, so thanks for this.

[CleanMachine1]

Says my review is required so here.

[sbrl]

Thanks, @marchersimon!

[owenvoke]

Looks good, just one minor comment.

[sbrl]

A great reference to refer people to when reviewing PRs!

[bl-ue]

LGTM

[bl-ue]

@SethFalco care to give this PR a review? If not, we can merge.

[SethFalco]

We're using title case for the h1 at the top, but then sentence case for the h2s?
It feels a bit inconsistent for me, any opinions? (More Info)

Some changes are subjective, so don't be afraid to dispute them.

The only one I'm somewhat firm on would be the 28 syllable clause. Try reading all of that in one breath!

[CleanMachine1]

Could the image(s) be made into a dark theme instead of light. How many people use dimmed/dark over black on white

[SethFalco]

Could the image(s) be made into a dark theme instead of light. How many people use dimmed/dark over black on white

To be honest, I find it unlikely that most use dark mode.
I think the default theme is best for the images.

I assume that'd be light theme, or system preferences. (But all major operating systems default to light, anyway.)

[CleanMachine1]

Its not a big deal. I'm fine with "default settings" being used.

[marchersimon]

I originally used dark theme, but @ bl-ue suggested to use the default one here. I understand the decision even tough a dark image in light mode isn't as bad as a light image in dark mode.

[marchersimon]

Merging, since there are already 5 approvals