Added documentation for visualiser and named access

[nic-barbara]

• Improved the docs for the visualiser to explain how to switch modes
• Made the controller the default mode, followed by trajectory. Passive dynamics are now last
• Added note in named access docstrings that MuJoCo IDs start at 0.

[codecov-commenter]

Codecov Report

Attention: 66 lines in your changes are missing coverage. Please review.

Comparison is base (32a25ed) 25.59% compared to head (9db2647) 25.47%.

Files	Patch %	Lines
src/LibMuJoCo/functions.jl	0.00%	48 Missing :warning:
src/LibMuJoCo/structs.jl	0.00%	16 Missing :warning:
ext/VisualiserExt/visualiser.jl	0.00%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #85 +/- ## ========================================== - Coverage 25.59% 25.47% -0.13% ========================================== Files 22 21 -1 Lines 5168 5161 -7 ========================================== - Hits 1323 1315 -8 - Misses 3845 3846 +1 ```

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