Describe the bug

It currently seems to be impossible to add a new public key to user's account using GHMyself. POST /user/keys is the relevant GitHub API endpoint.

To Reproduce
Read docs on GHMyself and observe the method not being present

Expected behavior
`GHMyself::addK

This is an umbrella issue to cover all the things we need regarding documentation for 2.0:

• Examples: #125
• Enable javadoc failures when running mvn verify (mvn javadoc plugin) #275
• Check missing javadoc, missing javadoc tags and internal and external links are working properly
  • api #267
  • core
  • formats/json-jackson
  • http/basic

1- hasOnlyOneElementSatisfying is deprecated, we need to use singleElement()

warning: [deprecation] hasOnlyOneElementSatisfying(Consumer<? super ELEMENT>) in AbstractIterableAssert has been deprecated
            .hasOnlyOneElementSatisfying(throwable -> {
            ^

For example:

assertThat(errorMessages)
            .hasSize(1)
            .hasOnlyOneElementSat