Adding docs for GPT API audits (#253)

* Adding docs for GPT API audits

* Updates to docs based on comments

* Fixes to audit docs to add clarity / amend spelling errors

* A few grammar fixes to GPT Audit Docs

docs/audits/deprecated-api-usage.md

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+# Avoid deprecated GPT APIs
+
+## Overview
+
+This audit ensures that the page does not use any deprecated Google Publisher Tag (GPT) APIs.
+Deprecated APIs are googletag methods or configuration options which are no longer maintained or documented. As these APIs 
+may be partly or wholly removed from GPT, their use is discouraged and unpredictable behavior could come from their 
+continued use.
+
+## Recommendations
+
+Observe the details of the audit to see which deprecated APIs the page is using and remove their usages.
+Many deprecated APIs will have a supported alternative which can be used instead.
+
+Check [the official GPT release notes](https://developers.google.com/publisher-tag/release-notes) for more information on
+deprecated APIs and suggested alternatives.
+
+## More information
+
+This audit checks console messages for the presence of any warnings or errors coming from GPT which are related to deprecated or 
+discouraged APIs.

docs/audits/gpt-errors-overall.md

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+# Fix GPT errors
+
+## Overview
+
+This audit ensures that no warnings or errors related to Google Publisher Tags (GPT) are present. Reducing errors can help 
+ensure that user experience is consistent and the page is tagged as intended.
+
+## Recommendations
+
+Observe the details of the audit to see more specific details on the errors affecting the page. 
+Run [Publisher Ads Audits for Lighthouse](https://developers.google.com/publisher-ads-audits)
+regularly when making changes to the webpage to minimize the introduction of new errors.
+
+Check [the official GPT best practices pages](https://developers.google.com/publisher-tag/guides/general-best-practices) 
+for more information on recommended usage of GPT on your page.
+
+## More information
+
+This audit checks console messages for the presence of any warnings or errors coming from GPT, excluding those related to other 
+audits.