docs: add alias to plugin add/install

[KristianDD]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

Currently they are in separate articles and there is no alias in articles

What is the new behavior?

There should be aliases in both articles.

Fixes
#3677

[miroslavaivanova]

run ci

[rosen-vladimirov]

btw I think the correct commit message should be
docs: add alias to plugin add/install

[miroslavaivanova]

run ci

[KristianDD]

Negotiated with QA and CLI team to merge with passing Travis CI.