Improve docs with practical migration info

[tbm]

It would be nice to cover some practical migration topics that aren't handled by ledger2beancount directly.

I can think of:

• envelope budgeting
• automated transactions
• virtual postings
• verification / validate (e.g. Zack's plugin)
• https://github.com/quentinsf/icsv2ledger

@zacchiro do you have other topics?

Although arguably this might be better in the main beancount docs (?)

[tbm]

@zacchiro asked why this would be in the l2b docs at all (rather than in e.g. the beancount docs) and I guess he's right.