Add documentation for Helidon Transactions.

[Tomas-Kraus]

Need document similar to https://github.com/Tomas-Kraus/helidon/blob/helidon-data-doc/docs/src/main/asciidoc/se/data.adoc for new Transaction API.

Also io.helidon.transaction package-info should contain few sentences from this document to allow better guadiance trough the API.

[Tomas-Kraus]

Copy-paste of some relevant comments from Helidon Data PR:

• Add relevant "see" annotations to Tx class javadoc as well to help guide the average new user
• Do we ever define and specify what a transaction is?
• Do we define or specify what a transactional method is, and how it is automatically executed?
• Tx.transaction methods:
  • Are there other kinds of transactions? Are they defined anywhere?
  • What does it mean to execute a task "as" a database transaction?
  • Do you mean "in" a database transaction?
  • What if the task has nothing to do with databases?
  • What if I'm not using a database?
• package-info: This is the entry point most new users will have to the Helidon Data transaction-related capabilities. This documentation must be comprehensive, starting with what a transaction is, what its invariants are, why you would use one, etc.
• module-info: This is the entry point for new users to what looks like a major feature. The documentation must be comprehensive here, starting with what a transaction is and why we define our own concept of it.
• TxLifeCycle: This interface does not describe the lifecycle of a transaction. It instead describes the contract for things that want to be notified of various events concerning transactions, plural.
• TxLifeCycle: What about suspend and resume? It sounds like there is a state diagram involved that all Helidon transactions must implement, but I don't know what it is. It must unite JTA transactions and...other? transactions? Elsewhere we've referred to "database transactions" but that is also undefined.

JTA module

• module-info: I don't understand how a module is a registry. -> can be described in a better way
• module-info: -> can be described in a better way