Docs: remove old content from 'developer_guide/'

[ramirezfranciscof]

This is a structure that was left over from the old documentation and needs to be removed, but first we need to check if all the content has been re-assigned in the new documentation. The files considered are:

• [x] design/changes.rst
• [x] core/modifying_the_schema.rst
• [x] core/extend_restapi.rst
• [x] core/transport_template.py (downloadable file in core/transport.rst)
• [x] core/transport.rst
• [x] core/internals.rst
• [x] core/caching.rst
• [x] plugins.rst (only index, can be deleted directly)
• [x] core.rst (only index, can be deleted directly)

Proposition:

Most of this content seems to be appropriate for the internals section:

• Global design: design/changes.rst
• RestAPI: core/extend_restapi.rst
• ~~DB schema: core/modifying_the_schema.rst~~
• ~~Entities: core/internals.rst~~
• ~~[Engine: core/caching.rst? I think it contains mostly info about the internals of the caching mechanism, but just in case it might be also considered altogether with this issue.~~

See comments for strikethrough sections.

The only exception being:

• core/transport.rst => How to add support for custom transport (also move core/transport_template.py).

Since the internals are not a priority right now, I wonder if we should just copy this pages into these new placeholders and leave it there until those sections are tackled so that we can remove the old structure from the repository. Alternatively we could put the files inside of internals/global_design.src folders if we don't want to display unfinished content in the documentation.