Updated links to VTK specification

[williamjsdavis]

Links to the VTK specification point to a page that does not exist any more:

http://www.vtk.org/VTK/img/file-formats.pdf

These links appear in:

1. The README (https://github.com/JuliaVTK/WriteVTK.jl/blob/d15119c2638b738437e2dd1c1787edfc541b9284/README.md?plain=1#L63)
2. The documentation index page (https://github.com/JuliaVTK/WriteVTK.jl/blob/d15119c2638b738437e2dd1c1787edfc541b9284/docs/src/index.md?plain=1#L56), and
3. The documentation for unstructured grids (https://github.com/JuliaVTK/WriteVTK.jl/blob/d15119c2638b738437e2dd1c1787edfc541b9284/docs/src/grids/unstructured.md?plain=1#L49)

I have replaced these links with the new appropriate link to the VTK specification in the documentation:

https://docs.vtk.org/en/latest/design_documents/VTKFileFormats.html

[codecov[bot]]

Codecov Report

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

Project coverage is 96.87%. Comparing base (d15119c) to head (a6b28bd).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #145 +/- ## ======================================= Coverage 96.87% 96.87% ======================================= Files 15 15 Lines 863 863 ======================================= Hits 836 836 Misses 27 27 ```

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[jipolanco]

Thank you!