doc: Adjust bitcoincore.org links

Minor adjustments to our https://bitcoincore.org/ links:

Mainly adding links of the Bitcoin Core Github mirror and Bitcoin Core website to the doxygen introduction. (See e.g. current master: https://dev.visucore.com/bitcoin/doxygen/index.html). Also removing the link to bitcoin.org, to not imply there is only one resource that educates about bitcoin.