task(doc): Flow diagrams reviewed

[stroncoso-quobis]

What type of PR is this?

• documentation

What this PR does / why we need it:

• It covers the detailed BYON flow, separated on three main use cases: notification channel, registration and call handling.
• It provides a necessary asset to attract developers and to fully understand the API mechanisms.

Which issue(s) this PR fixes:

Fixes #18 #32

Special notes for reviewers:

• Overseeds PR Documentation: FlowBYON #21
• Review PR Flow review - BYON #27
• Provides more detail regarding method headers, body and complimentary communications

Changelog input

taks(doc): Splitted diagrams and participant description included
task(doc): Detailed notification channel and internal messages
task(doc): General update of CallHandling flow
task(doc): Styles and fixed for call handling
task(doc): Separated FLOW for Notification channel API
task(doc): Included webhook option and some fixes
task(doc): fix title notification channel
task(doc): Flow detailed status and updated paths
task(doc): Updated sessionID and remove MoMs
task(doc): Improves notes on notification channels
task(doc): Included extra detail on PRACK and UPDATE

Additional documentation

Included exported UML diagrams for easier review

-- Registration / De-registration--

-- Notification channel --

-- Call handling (Originating / Terminating) --

[TEF-RicardoSerr]

LGTM. approved as per last meeting