Merge pull request chainer#7156 from asi1024/release-v7.0.0a1

Update version numbers

Merge pull request chainer#7035 from toslunar/bp-7027-support-sphinx-2

[backport] Support Sphinx 2.0 and use absolute path to support the latest RTD

Merge pull request chainer#6748 from asi1024/v6.0.0rc1

Bump version to v6.0.0rc1

Merge pull request chainer#6747 from hvy/v5.4.0

Bump version in Dockerfiles to v5.4.0

Merge pull request chainer#6383 from kmaehashi/release-v6.0.0b3

Bump version to v6.0.0b3

Merge pull request chainer#6382 from asi1024/v5

Bump version number to v5.3.0

Merge pull request chainer#6077 from asi1024/v6.0.0b2

Bump version to v6.0.0b2

Merge pull request chainer#6078 from kmaehashi/release-v5.2.0

Bump version number to v5.2.0

Merge pull request chainer#5743 from chainer/chainerx-docs

Build ChainerX documentation on ReadTheDocs

Merge pull request chainer#5758 from chainer/release-v5.1.0

Bump version to v5.1.0