Revamp the Parallel Processing section

Expand upon the Parallel Processing section by:
- enhancing the main section with specifics on segments, segment
configuration, and the rough outline of what's to come with sequencing
policies, threading modes and splitting/merging
- move and update the sequential processing section, by updating the
snippets and adding the new policy implementation
- add a "thread configuration" section. This section should explain how
to configure threads, that threads provides parallel power, how the
TEP and PSEP differ when it comes to threading and what pros/cons are
attached due to this
- update the multi-node processing section, by adding a piece about load
 balancing
- update the splitting and merging section, by providing explaining how
this is done in more detail

AxonIQ#189