Improved specification of device registers and operation (#150)

* Wording improvements, missing descriptions and notice for optional registers
* Reorganize document sections to reduce nesting and improve readability
* Add notes on optional registers and defaults
* Remove duplicated operation mode definitions
* Add backlinks to operation mode section
* Adopt requirement key words from RFC 2119
* Add links to individual register specs
* Move deprecated registers to separate section
* Remove speed mode from overview to emphasize deprecation
* Add device interface conceptual background

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Co-authored-by: glopesdev <g.lopes@neurogears.org>

Author: bruno-f-cruz