(towards #2543) Simplify Introduction documentation

[sergisiso]

• Added "warnings as errors" in the shpinx CI job

• Renamed "nemo" to "generic" in tutorials and lib folders.

• I removed lots of documentation that seem unneeded: two files where not used, some explanation e.g. how to use venvs is better with a link to an authoritative the source, how to install each dependency separately is probably not needed....

• Merged tutorials and examples sections and rewrote tutorials part

• I made termcolour and graphviz standard dependencies

• I started using sphinx tabs for alternative options, see image:

  

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.88%. Comparing base (8336dd6) to head (eb1b96d). Report is 12 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2770 +/- ## ========================================== + Coverage 99.87% 99.88% +0.01% ========================================== Files 356 356 Lines 49449 49446 -3 ========================================== + Hits 49385 49388 +3 + Misses 64 58 -6 ```

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[sergisiso]

@arporter @hiker This is ready for review. A bit unfortunately the linkcheck does not pass because the last commit updated a link to https://github.com/stfc/PSyclone/tree/master/tutorial/practicals/generic which won't exist until itself is merged, but you can check in the previous commit that the tests pass with the warning as errors flags.

[sergisiso]

@arporter This is ready for another review