[WEBSITE] Wait for bitcoind to be fully started before fetching the help

Summary:
The current behavior waits for the PID file to be created, but this is
not always enough. This diff adds a active wait until the RPC interface
is ready.

Test Plan:

./scripts/fetch-documentation.sh

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7331