[Add]: Deprecation policy

[imath]

What needs to be done?

We should clearly inform how we load deprecated code:

By default:

• Fresh installs: no deprecated code is loaded.

• Upgrade from a previous version: the 2 last versions deprecated code is loaded.

• Explain why we are using a 12.0 exception

• Explain how to disable deprecated code loading

• Explain how to force deprecated code loading

See: https://buddypress.trac.wordpress.org/ticket/9210

Handbook to add the page in

buddypress/docs/contributor

Tasks to accomplish

• This task has been added to the "Todo" panel of the Documentation project.
• A PR has been submitted to the BuddyPress GH repository, it has been linked to this issue/task & the task was moved to the "In progress" panel of the Documentation project.
• The PR has been converted to a SVN patch and committed to the BuddyPress SVN repository & the task was moved to the "Done" panel of the Documentation project.

[imath]

Here's a starting point, the developer note I wrote during 11.0 dev cycle:
https://bpdevel.wordpress.com/2022/11/20/the-way-buddypress-loads-deprecated-code-will-change-in-version-11-0-0/