Make Migrations page on docs

[untroublua]

Add migrations page for documentation with example and tips

[anthony0br]

Thank you for your contribution!

Some comments from myself:

• The term "database" in this context is a bit vague - could you replace it with "schema"? Also, note that migrations can do more than just adding or removing values - they can take an existing format and change it into something else, or fix inconsistencies in data!
• Please run the code through StyLua to make it prettier and make sure your comments don't go over 80 lines. If in doubt, consult the Roblox Lua Style Guide.
• You don't need [1], [2], [3] etc as indexes
• Could we have a better explanation of backwards compatibility and how migrations differ compared to reconciliation?
• Also not sure on the use of Sift - I like it, but is this too much to introduce at once?

I'll look into merging next week - I don't have time to fix all of the issues listed above at the moment, so further contributions are welcome.