[skip-ci][hist] Document architecture for initial set of classes

hist/histv7/doc/CodeArchitecture.md

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+# Code Architecture
+
+This document lists all classes, describes their responsibilities, and details how they fit together.
+
+## Core Classes
+
+These classes are involved when the user calls `Fill`.
+The list is ordered "bottom-up" in terms of functionality.
+
+### `RRegularAxis`, `RVariableBinAxis`, `RCategoricalAxis`
+
+These classes implement a concrete axis type.
+Their main task is to compute the linearized index for a single `Fill` argument:
+```c++
+RLinearizedIndex ComputeLinearizedIndex(double x);
+```
+The `bool` is used to indicate if the return value is valid.
+For example, the argument may be outside the axis with the underflow and overflow bins disabled.
+`RLinearizedIndex` is a simple struct with a `std::size_t index` and `bool valid`.
+It is chosen over `std::optional` because it unifies the return value construction:
+If outside the axis, the validity is just determined by the member property `fEnableFlowBins`.
+
+### `Internal::RAxes`
+
+This class is responsible for managing a histogram's axis configuration.
+It stores the axis objects as a `std::vector` of `std::variant`s.
+Objects of this class are used internally and not exposed to the user.
+Relevant functionality is available through user-facing classes such as `RHistEngine`.
+
+The main function is
+```c++
+template <typename... A>
+RLinearizedIndex ComputeGlobalIndex(const std::tuple<A...> &args) const;
+```
+that dispatches arguments to the individual `ComputeLinearizedIndex` and combines the results.
+If any of the linearized indices is invalid, so will be the combination.
+
+### `RHistEngine`
+
+This class combines an `RAxes` object and storage of bin contents in a `std::vector`.
+During `Fill`, it calls `RAxes::ComputeLinearizedIndex` and then updates the bin content.
+In contrast to `RHist`, this class supports direct concurrent filling via `FillAtomic`.
+
+### `RHistStats`
+
+Manages the (global) histogram statistics, such as the number of entries.
+It also keeps statistics of the unbinned values for each dimension.
+
+### `RHist`
+
+This class combines `RHistEngine`, with its axes and bin contents, and `RHistStats`.
+During `Fill`, it delegates to `RHistEngine::Fill` but also updates the histogram statistics.
+
+## Classes for Weighted Filling
+
+### `RDoubleBinWithError`
+
+A special bin content type that also accumulates the sum of weights squared.
+It can be used as a template argument to `RHistEngine` and `RHist`.
+
+### `RWeight`
+
+A wrapper `struct` for a single `double` value, used for weighted filling to distinguish its type.
+Objects of this type are passed by value.
+
+## Auxiliary Classes
+
+### `RBinIndex`
+
+A single bin index, which is just an integer for normal bins.
+`Underflow()` and `Overflow()` are special values and not ordered with respect to others.
+Objects of this type are passed by value; most notably to `GetBinContent` and `SetBinContent`.
+
+### `RBinIndexRange`
+
+A range of `RBinIndex` from `begin` (inclusive) to `end` (exclusive).
+The class exposes an iterator interface that can be used in range-based loops.