No CETL_NODISCARD for most of the public api

[serges147]

No CETL_NODISCARD for the most of public api (issue #350), but private (aka detail) stuff stays under strict [[nodiscard]] policy for all non-void returning functions.

Also:

• Less NOSONAR-s by switching some internal void*-s → cetl::byte*-s.
• Eliminated IMultiplexer at CAN - no use of it; but will be at UDP.
• More docs for public api.
• Fixed Build Status and Sonar Cloud badges at README.md.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
1 Accepted issue

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[lydia-at-amazon]

nit: Title of PR: No CETL_NODISCARD for the most of public api --> No CETL_NODISCARD for most of the public api

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
1 Accepted issue

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud