Improve the bitcoin.conf instructions in init.md doc

[epiccurious]

Update the init.md instructions about generating an example bitcoin.conf file.

Additionally, the share/examples/bitcoin.conf placeholder file says to follow the instructions in contrib/devtools/README.md, which was confusing to me since that README talks about many scripts. It would be helpful to point out to the user which section to refer to.

This issue includes an accompanying PR #30154.

Background

• Two years ago, share/examples/bitcoin.conf was replaced with a placeholder file. To see an example bitcoin.conf, the user now runs the contrib/devtools/gen-bitcoin-conf.sh script, which replaces the placeholder file with the parsed contents of bitcoind --help.

• The instructions in init.md about an example bitcoin.conf haven't changed significantly since they were added almost 10 years ago.