[6pt] memtx description in Storage engines section: scoping

[veod32]

Related to #1632

Discussed with @Mons.

Topics to be covered in broader description of how memtx works can be the following:

• memory model (allocation -- in one thread; the small memory allocator)
• механизм записи всего этого на диск -- persisting data on disk (xlog, snapshots)
• доступ к данным в виде индексов -- accessing data (indexes - как минимум рассказать про основной - древовидный)

Revised content from presentations and trainings about Tarantool can be used.

Draft storyline can be the following:

• All data is in memory
• Access to data is from one thread
• Changes are being written in the Write Ahead Log (WAL)
• Indexes are build to access the data
• Data snapshots are taken periodically
• WAL is replicated