[Docs] adjust gendocs.sh to documents by convention structure.
[Docs] adjust gendocs.sh to documents by convention structure.

Summary:
This will copy assets to the generated documentation folder and adjust the paths to the assets accordingly in the generated .html files.

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Closes https://github.com/google/material-components-ios/pull/133
GitHub author: peterfriese

!![[https://github.com/google/material-components-ios/pull/133 | This is a child of GitHub pull request #133]]!!.

Reviewers: #mdc_ios_owners, github-bot

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D199

willlarche/material-components-ios,willlarche/material-components-ios,sigito/material-components-ios,samnm/material-components-ios,brianjmoore/material-components-ios,samnm/material-components-ios,willlarche/material-components-ios,material-components/material-components-ios,brianjmoore/material-components-ios,randallli/material-components-ios,Eccelor/material-components-ios,chriscox/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,brianjmoore/material-components-ios,samnm/material-components-ios,Eccelor/material-components-ios,willlarche/material-components-ios,randallli/material-components-ios,Eccelor/material-components-ios,sigito/material-components-ios,willlarche/material-components-ios,randallli/material-components-ios,sigito/material-components-ios,material-components/material-components-ios,brianjmoore/material-components-ios,randallli/material-components-ios,randallli/material-components-ios,chriscox/material-components-ios,brianjmoore/material-components-ios,Eccelor/material-components-ios,chriscox/material-components-ios,chriscox/material-components-ios,Eccelor/material-components-ios,randallli/material-components-ios,samnm/material-components-ios,sigito/material-components-ios,sigito/material-components-ios,samnm/material-components-ios,material-components/material-components-ios,chriscox/material-components-ios