Update effective-forms.md

Replaced with a ton of content from "Designing transaction" in the Gov.uk Service Manual (https://www.gov.uk/service-manual/user-centred-design/designing-transactions.html), which uses the Open Government License v2.0 (http://www.nationalarchives.gov.uk/doc/open-government-licence/version/2/.
Update effective-forms.md

fixed image links
Update effective-forms.md

Updated more links
Update effective-forms.md

Added information about communicating errors
Update effective-forms.md

content edits about validation and helping users
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Updated Table of Contents
Update effective-forms.md

adjusted attribution language
Update effective-forms.md

copy edits for line 24. not loving it yet.
Update Primary and Secondary buttons content
Remove paragraph
Remove statement

@benguhin and @ohsk thought this was unnecessary, and that "submit" is
actually a legitimate label in some cases. Could supplement if there
are better options!
Adjust column proportion
Adjust column proportion
Merge pull request #161 from ohsk/gh-pages

Small tweaks to UI Toolkit > Buttons page
Merge pull request #160 from Scotchester/gh-pages

Make color page responsive; remove date from banner
Merge pull request #162 from Scotchester/gh-pages

Fix grid demo styling
commented out UI toolkit > Pagination page for R1
Edited content, colors of Grid page
styling tweaks to Grid
Tweaked height of color swatches (#167). Began reformatting Guides pages.