docs/packages/contents.rst:
Fix documentation on changes between PkgInfoV1 and PkgInfoV2

docs/packages/contents.rst, docs/repositories/management_repository.rst:
Include all JSON schema files from docs/schema.

tox.ini:
Export all JSON schema files to docs/schema.

docs/packages/contents.rst:
Add documentation about Package and include PackageV1 JSON schema.

docs/packages/contents.rst:
Add documentation about .PKGINFO files, including JSON schema for
PkgInfo.

docs/packages/contents.rst:
Extend sphinx documentation with information on .BUILDINFO files.

docs/packages/contents.rst:
Add documentation about the contents of a package and a section about
.MTREE files and the JSON schema that can be used to validate parsed
input with.

docs/packages/signatures.rst:
Add documentation about the supported signature files for packages.