docs(token): document the token format

[nobe4]

Add details on the format of OpenBao tokens, it should hopefully add clear documentation as to one can detect tokens.

The body's format was inferred from:

• openbao/vault/token_store.go

  Lines 72 to 74 in 1800244

  	// TokenLength is the size of tokens we are currently generating, without
  	// any namespace information
  	TokenLength = 24

• openbao/vault/token_store.go

  Lines 997 to 999 in 1800244

  	entry.Accessor, err = base62.Random(TokenLength)
  	if err != nil {
  	return err

Resolves #369

Target Release

1.14.7

[cipherboy]

Thank you!

[cipherboy]

@nobe4 Mind adding DCO git commit --amend --signoff? Thanks!

[nobe4]

@cipherboy, trying to do it at the moment, first time doing this 😬

[nobe4]

The DCO page was really helpful 👏
https://github.com/openbao/openbao/pull/372/checks?check_run_id=26370274219