Migration guide

[VictorKoenders]

To be merged after #481

Adds a migration guide. This is also automatically included in the docs.rs documentation.

Some links do not work yet, as some of the changes are not published:

• the config changes in Moved Configuration::standard() and ::legacy() to the config module #481
• The Encode and Decode manual implementation guide in Update documentation #480

Rendered

[codecov-commenter]

Codecov Report

Merging #482 (4a26207) into trunk (a041810) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            trunk     #482   +/-   ##
=======================================
  Coverage   66.24%   66.24%           
=======================================
  Files          38       38           
  Lines        2708     2708           
=======================================
  Hits         1794     1794           
  Misses        914      914           

Impacted Files	Coverage Δ
src/lib.rs	100.00% <ø> (ø)

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