[Chore] Upgrade guide tracker for released and unreleased features

.github/PULL_REQUEST_TEMPLATE.MD

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 - [ ] This PR follows the [Contribution Guide](https://github.com/Sitecore/jss/blob/dev/CONTRIBUTING.md)
 - [ ] Changelog updated
+- [ ] Upgrade guide entry added
 
 ## Description / Motivation
 <!--- Describe your changes in detail -->

UPGRADING.md

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+# Upgrade guides
+
+This document includes the links to upgrade guides for different versions of JSS apps.
+Each version contains the list of files, dependencies and environment variables that need to be added, updated or removed. Changes are grouped by templates and add-ons in each corresponding page.
+Work-in-progress items are listed in [Unreleased](./docs/upgrades/unreleased.md).
+
+
+## Upgrade paths for 21.x versions
+- [Upgrade from JSS 21.6 to 21.7](./docs/upgrades/21.x/21.7.md) 
+- [Upgrade from JSS 21.5 to 21.6](./docs/upgrades/21.x/21.6.md)

docs/upgrades/21.x/21.6.md

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+## Upgrade to JSS 21.6 from 21.5
+
+* Find the list of changes in Sitecore documentation:
+  * [XM Cloud](https://doc.sitecore.com/xmc/en/developers/jss/216/jss-xmc/upgrade-jss-21-5-next-js-apps-to-version-21-6.html)
+  * [SXP / OnPrem](https://doc.sitecore.com/xp/en/developers/hd/21/sitecore-headless-development/upgrade-jss-21-5-next-js-apps-to-version-21-6.html)
+

docs/upgrades/21.x/21.7.md

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+## Upgrade to JSS 21.7 from 21.6
+
+* Find the list of changes in Sitecore documentation:
+  * [XM Cloud (to be added when published)](https://doc.sitecore.com/xmc/en/developers/jss/latest/jss-xmc/upgrade-jss-21-6-4-next-js-apps-to-version-21-7.html)
+  * [SXP / OnPrem](https://doc.sitecore.com/xp/en/developers/hd/21/sitecore-headless-development/upgrade-jss-apps-to-jss-21-7.html)
+

docs/upgrades/unreleased.md

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+## Unreleased
+
+### nextjs
+
+### nextjs-xmcloud
+
+### nextjs-sxa
+
+### nextjs-multisite
+
+### nextjs-styleguide
+
+### react
+
+### angular
+
+### vue