[Docs] Change ordering of transaction isolation levels

Description

source: https://docs.yugabyte.com/latest/architecture/transactions/isolation-levels/#root

We begin the section by listing out 4 transaction isolation levels in decreasing order of strictness. Where possible, subsequent sections should follow this order to make it easier for readers to follow. For example, rewrite the second paragraph to read:

YugabyteDB supports three transaction isolation levels - Serializable, Snapshot (which maps to the SQL isolation level REPEATABLE READ), and Read Committed. Thus, YugabyteDB supports the three strictest of the above four isolation levels. These isolation levels are described below:

Serializable Isolation guarantees that transactions run in a way equivalent to a serial (sequential) schedule.

Snapshot Isolation guarantees that all reads made in a transaction will see a consistent snapshot of the database, and the transaction itself will successfully commit only if no updates it has made conflict with any concurrent updates made by transactions that committed since that snapshot.

Read Committed Isolation guarantees that each statement will see all data that has been committed before it is issued (note that this implicitly also means that the statement will see a consistent snapshot). Also, this isolation level internally handles read restart and conflict errors. In other words, the client doesn't see read restart and conflict errors (barring an exception). Details are in the READ COMMITTED isolation level section.