Explain private keys and account funding

[miohtama]

Some basics about private keys and using ETH as a gas fee. This information is unknown to many new developers.

What was wrong?

People do not know how to use private keys in development.

How was it fixed?

Explain the basics in the documentation.

Cute Animal Picture

((...))         ((...))         ((...))           L...L  
( o o )         ( x x )         ( O O )          < o o >
 \   /           \   /           \   /            \   /
  ^_^  cp97       ^_^  cp97      (`_`)  cp97       ^_^  cp97

[fselmo]

Thanks for the much-needed improvement / clarification. Just had some minor typos / suggestions. I can make those if you're good with it and add a doc newsfragment... lmk 👀

[miohtama]

@fselmo resolved spell check - thank you!

[fselmo]

rebased and consolidated commits 👍🏼