expand documentation about seed format (electrum mnemonic words), detail specific version numbers

[SomberNight]

@chris-belcher asked on IRC for more detailed documentation about the seed format.

We currently have seedphrase.rst but it only details how to calculate the version number of a seed, and lists registered version numbers.

We should either expand that doc, or add another, that details the schemes that correspond to specific version numbers, i.e. how to go from seed words to bip32 chains (and what to use those chains for).

Note that there are unit tests here: https://github.com/spesmilo/electrum/blob/8d7370d897314d8542906aecc6a45cc949651f77/electrum/tests/test_wallet_vertical.py#L106
we could link to those in the doc, resolving potential ambiguities

Related:
https://github.com/spesmilo/electrum/blob/8d7370d897314d8542906aecc6a45cc949651f77/electrum/mnemonic.py#L159
https://github.com/spesmilo/electrum/blob/8d7370d897314d8542906aecc6a45cc949651f77/electrum/keystore.py#L975

[chris-belcher]

One of Electrum's selling points is "No Lock-In - Exportable keys to other clients." That also justifies working on this documentation, because maybe other software will end up supporting electrum seed phrases and users could use those to recover their funds too.