Mention Certbot `show_account` subcommand on Account ID page

[osirisinferi]

Recently, a new subcommand was added to Certbot: show_account

While it was mainly intended to show stuff like email contact info of the account, the account ID is added to the output too. Therefore, for users running certbot 1.23.0 or newer could use this simple subcommand to fetch their account ID instead of the requirement to open JSON files.

While adding this info I also added a link to the Certbot homepage on the first mention of Certbot. Can't hurt methinks.

Note: Certbot 1.23.0 hasn't been released yet. Therefore, I suggest waiting with merging this PR, if accepted, until the next release of Certbot.

[osirisinferi]

FYI: Certbot 1.23.0 has been released recently! :slightly_smiling_face:

[pgporada]

@osirisinferi This is a great idea and I'm sorry I missed it FOR AN ENTIRE YEAR.