[Docs] Upgrade path from Magento 2.1 to 2.3

[buskamuza]

Document upgrade path from Magento 2.1 (PHP 7.0) to Magento 2.3 (PHP 7.1/7.2).

[shrielenee]

Since 2.1 supports PHP 7.0 and 2.3 only supports 7.1 and 7.2, upgrading from 2.1 to 2.3 requires some special steps and is not super easy.

Both command line and web wizard instructions for upgrading from 2.1 to 2.3 need to be reviewed and modified. This will likely also require release notes.

All of the work for PHP 7.2 to be fully supported may not be completed by alpha, though using Composer it can be installed and will be compatible. We fully expected and planned to get the upgrade instructions ready for all versions by alpha, but given that not all the information is confirmed yet it is expected that docs will not be ready for alpha.

I am speaking with Dima about getting his support for completing this documentation, likely post-alpha.

[shrielenee]

Dima and I are working together on the upgrade path, as he is already working on upgrading from 2.2 to 2.3.

Dima's Jira issue: https://jira.corp.magento.com/browse/MAGEDOC-2812

Applicable pages to revise include:

• http://devdocs.magento.com/guides/v2.3/comp-mgr/prereq/prereq_compman.html
• http://devdocs.magento.com/guides/v2.3/comp-mgr/cli/cli-upgrade.html
• http://devdocs.magento.com/guides/v2.3/comp-mgr/upgrader/upgrade-checklist.html
• http://devdocs.magento.com/guides/v2.1/comp-mgr/bk-compman-upgrade-guide.html
• http://devdocs.magento.com/guides/v2.2/migration/bk-migration-guide.html
• http://devdocs.magento.com/guides/v2.2/comp-mgr/upgrader/ce-ee-upgrade-start.html
• http://devdocs.magento.com/guides/v2.2/comp-mgr/upgrader/upgrade-start.html (and steps)
• http://devdocs.magento.com/guides/v2.2/install-gde/install/cli/dev_options.html
• http://devdocs.magento.com/guides/v2.2/install-gde/install/cli/dev_update-magento.html