Improve documentation: byte layouts, release pages, See also, dark-mode images (#5906)

- Make the iterator range illustrations legible in dark mode (prefers-color-scheme
  rules inside the SVGs).
- Add a "Byte layout" section with a color-coded byte table to each binary format
  page (BJData, BON8, BSON, CBOR, MessagePack, UBJSON), and highlight the smallest
  size per file in the comparison table.
- Add "See also" sections to the overview pages that lacked one.
- Split the release notes into one page per release, grouped by minor version on
  the overview page, with an "API changes" section per release generated from the
  API pages' version histories (hooks/api_changes.py).
- Fact-check the release notes and version histories against the release tags and
  correct wrong or misleading entries (e.g., json_serializer 2.1.0, merge_patch
  3.1.0, items, count, operator[], to_bjdata, std::string_view key support).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/home/releases/v3.0.0.md

@@ -0,0 +1,20 @@
+# v3.0.0
+
+Released on 2017-12-17.
+
+First 3.x release — a major release with breaking changes (see the
+[migration guide](../../integration/migration_guide.md)).
+
+- Introduces user-defined [exceptions](../exceptions.md) (`json::exception` and subtypes, each with an
+  identifier).
+- Adds a non-throwing [`accept`](../../api/basic_json/accept.md) function and an `allow_exceptions` flag
+  for [`parse`](../../api/basic_json/parse.md), and an [`update`](../../api/basic_json/update.md) function to
+  merge objects.
+- Adds streaming for CBOR and MessagePack and allows storing NaN/infinity.
+- Non-UTF-8 strings now throw on serialization, and the iterator category changed to bidirectional.
+
+[Full release notes](https://github.com/nlohmann/json/releases/tag/v3.0.0).
+
+## API changes
+
+<!-- api-changes -->