API Manual & Improve doc

Some functional or runtime-specific details cannot be placed in the calls' doc comments, even if wallet developers need them to understand how to use the calls. This is why I added a new documentation file, in a more tutorial fashion than the automated API reference.

I also harmonized the tabbing, because fmt seems to miss some files.

docs/api/manual.md

+# Manual for wallet developers
+
+This functionnal documentation presents how wallets can interact with the blockchain.
+It is intended to complete the [runtime calls documentation](./runtime-calls.md) in a runtime-specific way to fit the real needs of wallet developers.
+
+Only ĞDev is covered for now.
+
+## Notations
+
+1 ĞD = 100 units
+
+## Account existence
+
+An account exists if and only if it contains at least the existential deposit (2 ĞD).
+
+## Become member
+
+Only use `identity` pallet. The `membership` calls are disabled.
+
+1. The account that wants to gain membership needs to exists.
+1. Any account that already has membership and respects the identity creation period can create an identity for another account, using `identity.createIdentity`.
+1. The account has to confirm its identity with a name, using `identity.confirmIdentity`. The name must be ASCII alphanumeric, punctuation or space characters: ``/^[-!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~a-zA-Z0-9 ]{3,64}$/`` (additionally, trailing spaces and double spaces are forbidden, as a phishing countermeasure). If the name is already used, the call will fail.
+
+## Revoke an identity
+
+Revoking an identity makes it lose its membership, hence UD creation and governance rights. Orher data such as balance will remain.
+
+This feature is useful in case the user has lost their private key.
+
+### Generate the revocation payload
+
+1. Scale-encode the revocation payload, that is the concatenation of the 32-bits public key and the genesis block hash.
+2. Store this payload and its signature.
+
+### Effectively revoke the identity
+
+1. From any origin that can pay the fee, use `identity.revokeIdentity` with the revocation payload.