docs: Widevine CDM plugin has more requirements in >=v1.8.0

[alexanderturinske]

Describe changes in Widevine CDM plugin based on new widevine requirements

Relates to: #12427
Relates to: #10806

Checklist

• PR description included and stakeholders cc'd
• relevant documentation is changed or added
• PR title follows semantic commit guidelines

Notes: Add docs on new widevine requirements.

[welcome]

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[alexanderturinske]

This was only a document change and all the errors in the console look unreleated

[MarshallOfSound]

@alexanderturinske Sorry for taking a while to look at this, it looks like this change has picked up some conflicts, can you resolve those 👍

[alexanderturinske]

Done

[alexanderturinske]

@MarshallOfSound Plz review

[codebytere]

@alexanderturinske could you please add release notes to your PR body and fix the following lint errors?

2018-10-19T01:20:02.6807378Z 
2018-10-19T01:20:04.3796501Z docs/tutorial/testing-widevine-cdm.md�
2018-10-19T01:20:04.3798198Z   11:1-17:4 Warning: Missing code-language flag  fenced-code-flag  remark-lint

[release-clerk]

Release Notes Persisted

Add docs on new widevine requirements.

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉