docs: add v0.18 migration guideline (ignite#1599)

* Add v0.18 migration

* Tabs to spaces

* add a description of migration requirements and context of why needed

* Update docs/migration/index.md

blockchain template

Co-authored-by: Denis Fadeev <[email protected]>

* Apply suggestions from code review

submitting changes to clarify reason and actions

* Update docs/migration/index.md

add missing blank line

* Intro

* Add go.mod

* Update docs/migration/index.md

Co-authored-by: Barrie Byron <[email protected]>

* Update docs/migration/index.md

Co-authored-by: Barrie Byron <[email protected]>

* Update docs/migration/index.md

Co-authored-by: Barrie Byron <[email protected]>

* Update docs/migration/index.md

Co-authored-by: Lucas Bertrand <[email protected]>

* remove extra ``go

* NewClientProposalHandler

* Update docs/migration/index.md

Co-authored-by: Tobias Schwarz <[email protected]>

* Add app.appCodec as an argument

* Keep `tmjson`

* Move `go.mod` higher

* BandChain support

* If mars is an IBC-module

Co-authored-by: Barrie Byron <[email protected]>
Co-authored-by: Barrie Byron <[email protected]>
Co-authored-by: Lucas Bertrand <[email protected]>
Co-authored-by: Tobias Schwarz <[email protected]>
Co-authored-by: Danilo Pantani <[email protected]>