Signed-off-by: David Bauer <mail@david-bauer.net>

This allows us to organize the TOC a bit better by adding sections per
major version. We can even increase the maxdepth to 2 now, which looks
great in my opinion.

In addition, the full list of releases is not shown in the sidebar
anymore when viewing a completely different part of the documentation,
which took up more than half of the total sidebar entries.

With older versions of the RTD theme, paragraphs in nested lists would
lead to inconsistent spacing. This has been fixed, so we can remove our
workaround to use line blocks instead of paragraphs.

2 spaces is the most common indentation width used in the docs; adjust
the rest for consistency.

Also change .editorconfig accordingly.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit d7f9e17f)

I mixed up two separate improvements into one, this change aims to
clarify them based on a suggestion and conversation with @T-X.

Closes #1979
Closes #2062

(cherry picked from commit 582d2d69)

Closes #1979
Closes #2062

None of the other release pages have a "v" before the version number.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 5a7ac058)

(cherry picked from commit 4bfcd6ae)

Resolves #1937

(cherry picked from commit 1566324f)

(cherry picked from commit f1f188f8)

respondd address

(cherry picked from commit c612dfba)

(cherry picked from commit 1e66cd90)

Uncovered by sphinx-build:
docs/releases/v2018.2.1.rst:19: WARNING: Too many autonumbered footnote references: only 0 corresponding footnotes available.
docs/releases/v2018.2.1.rst:19: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:26: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:30: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:34: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:35: WARNING: Unknown target name: "noibss".