Create enforcement.md - Encryption Enforcement topic

[Andriciuc]

Writing a general topic regarding Encrpytion Enforcement from pg_tde perspective, requires heavy review.

[codecov-commenter]

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[dutow]

Now my generic problem with this PR is that this new page doesn't provide any additional information compared to the GUC page. In fact, it provides less, as the GUC page at least explains to an extent what are the different allowed scopes of the variable.

Jan asked for scenarios to be documented, I guess he meant things like:

• Specific example how to enforce encryption to the entire server
• Specific example how to enforce encryption to only a single database with ALTER DATABASE SET
• Example that superusers can override the variable at session level, so that they can create non encrypted tables in that session (the usecase I described on slack)
• Maybe also an example of enforcing encryption to a specific user with ALTER USER

Which is basically adding actual examples to the "Use the following techniques..." section

[Andriciuc]

Added a couple of simple examples to encryption enforcement for user, database and global enforcement using enforce_encryption

Added What does enforcement do (for new users)

Added that you can override enforcement with details possible by superusers only

Updated variables with minor linting and added a link to encryption enforcement for pg_tde.enforce_encryption