docs for celestia-app app version upgrades

[rootulp]

Context

• celestia-app app version v2 will be coordinated through a block height.
• celestia-app app version >= v3 will be coordinated on a rolling basis via validators signalling for which version they support.

Problem

Validators don't have a page to reference on how to upgrade

Proposal

This can wait until we're closer to releasing celestia-app app version v2 but we likely want a page for app version upgrades. The page should describe:

• what validators need to do to upgrade to v2
• what validators should expect to upgrade to >= v3

cc: @cmwaters