Restructure Source Repository

With the number of supported servers and migrations growing on every release, the current "one level" structure of the source repository needs to be reworked properly, not only to make it easier to understand the code, but also to make it easier to add new server and/or migrations.

The new structure should include the following placeholders:

• dist - where all distribution formats should be placed
• servers - where all server modules should be placed
• migrations - where all migration modules should be placed, grouped by target server
• docs/user-guides/migrations - where all migration user guides should be placed, yet grouped by target server

Considering the design above, the concrete changes should be applied:

• build should be moved to dist/standalone
• each xxx-feature-pack should be moved to dist/feature-packs/xxx
• each xxx-to-yyy migration module should be moved to migrations/yyy/xxx
• each xxx server module should be moved to servers/xxx
• each docs/user-guides/migrations/xxx-to-yyy migration user-guide should be moved to docs/user-guides/migrations/yyy/xxx

Also, the restructure should not change any Maven artifact ids, and all docs should be updated to reflect the changes applied, same for testsuite scripts.