Update "Start New Genesis Ledger" with advanced use of `--ledgerfile`

[mDuo13]

The documentation on how to Start a new Genesis Ledger in Stand-Alone Mode needs some updating. Notable details about starting a new stand-alone ledger that should be accounted for:

• Which amendments are enabled in the default ledger got more complicated now that amendments have default votes.
• The difference between [amendments] and [feature] stanzas
  • [amendments] is deprecated, ignored if you already have voting data in the wallet.db file, but can give more control over starting amendments
  • [feature] can "force-on" a feature, which shouldn't be used in network mode, with an explanation why
• Maybe also mention some concerns for starting a ledger that will be used as part of a network (private, dev, or sidechain), like:
  • the configurable amendment waiting time parameter which can be set for dev networks (e.g. Devnet uses I think a 30-minute timer instead of the default 2 weeks)
  • fee settings (transaction cost and reserves) and the legacy/new formats of the FeeSettings ledger entry (the new format only gets applied when there's a fee voting change after the amendment goes live)
• Mention using --ledgerfile to load a totally custom genesis ledger, which can do a bunch of stuff (described below)

The --ledgerfile {filename.json} commandline parameter to rippled can be used for advanced purposes, such as:

• setting a custom list of enabled amendments
• allocating initial XRP in a genesis ledger to custom account(s)

This comment has an example including a sample file.