Installation steps for impact framework is inconsistent across github and docs

[srini1978]

Couple of points I noticed:

1. The installation of impact framework steps is talking about yarn whereas the github page is talking about npm. We need to keep these 2 consistent.
2. The link https://github.com/Green-Software-Foundation/if#alternate-version which documents the alternate version needs to be checked. It did not work for me. Also if you do intend to keep this , recommend you name the first installation steps as one that uses npm and the second one as the one that uses yarn.

[jmcook1186]

Thanks @srini1978 - good point, @narekhovhannisyan and I discussed it and think we should favour npm over yarn

We will:

• remove the "alternate version" instructions
• remove the yarn lock files from all our repositories
• check all other related docs for yarn-based instructions.

[jmcook1186]

Closing as complete