Docs build error in DID Method crates

[clehner]

e.g.: https://docs.rs/crate/did-web/0.1.0/builds/370618
Need to set required features for the docs builds - or make the crates build with default features.

[clehner]

DID method crate updates published (#403 (comment)); docs built successfully:

• https://docs.rs/did-ion/0.1.0/did_ion/
  https://docs.rs/crate/did-ion/0.1.0/builds/519393
• https://docs.rs/did-webkey/0.1.1/did_webkey/
  https://docs.rs/crate/did-webkey/0.1.1/builds/519396
• https://docs.rs/did-method-key/0.1.3/did_method_key/
  https://docs.rs/crate/did-method-key/0.1.3/builds/519394
• https://docs.rs/did-ethr/0.1.1/did_ethr/
  https://docs.rs/crate/did-ethr/0.1.1/builds/519398
• https://docs.rs/did-onion/0.1.2/did_onion/
  https://docs.rs/crate/did-onion/0.1.2/builds/519397
• https://docs.rs/did-web/0.1.2/did_web/
  https://docs.rs/crate/did-web/0.1.2/builds/519403
• https://docs.rs/did-tz/0.1.2/did_tz/
  https://docs.rs/crate/did-tz/0.1.2/builds/519402
• https://docs.rs/did-pkh/0.1.1/did_pkh/
  https://docs.rs/crate/did-pkh/0.1.1/builds/519512