Change Log / Ray Tracing in One Weekend

From our last official v3.2.3 release (three and a half years ago!), this major release includes all

changes in the v4.0.0-alpha.1 and v4.0.0-alpha.2 releases, plus the changes listed immediately

below. Generally, this represents a large overhaul of all three books and their code, and will

require large changes to any code you've based on the prior v3.2.3 version. Going forward, we plan

to avoid such massive, long-running development branches, at the expense of more frequent minor and

major releases.

There's still a fair amount of work remaining on book three, which we'll work on after this release.

- New -- Add explicit std:: namespacing almost everywhere (#1487)

- Change -- Use delegating constructors where helpful (#1489)

- Change -- Standardized our use of `begin`/`end` standard C++ iterators (#1551)

- Fix -- CSS reformatting and fixes (#1567)

- Fix -- Add workaround for image and figure captions using latest Markdeep versions (#1583)

- New -- Add DOCTYPE declaration to all Markdeep documents (#1566)

- Delete -- Remove debug output code from `constant_medium::hit()` function (#1495)

- Change -- Convert `perlin` class to use static arrays instead of dynamically allocated (#1483)

- Fix -- Workaround Markdeep issue for code listings with tag-like tokens (#1463)

- Change -- Simplified the `onb` class, and renamed or deleted functions (#1080)

- Change -- Many small updates following walkthrough of book 3 (#988, #1317)

- Change -- Use plain array for `estimate_halfway` program (#1523)

- Change -- Refactored the ONB class to remove unused methods and generally simplify (#1088)

- Change -- Use `ICD(d)` instead of `f(d)` for inverse cumulative distribution for clarity (#1537)

- Fix -- Add missing signature updates for `material::scatter()` functions

- Fix -- Avoid `hittable_list` of lights in book until code is ready (#1318)