Do not background the process while serving

Summary:
It doesn't make any sense to do this since:

• The user is forced to kill the process manually when they are done.
• The terminal instructs the user to Ctrl-C to kill the server, when in fact it does not work.

Test Plan:

make serve

Ctrl-C should kill the server as expected.
Verify the process is no longer running:

ps aux | grep jekyll

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6591