Simplify and enhance using examples in documentation

[mikemckiernan]

The naive approach in 22.04 for repurposing the example notebooks was to copy the examples directory into the Sphinx source directory and build. This simple approach was OK for some repositories and not OK for many.

The major downside for examples directories that have subdirectories, is that we needed to use README.rst with toctree directives or the notebooks were orphaned from the navigation. Using RST is too inconvenient and we need a way to repurpose the README.md files that already exist.

After evaluating AutoStructify and sphinx-external-toc, I think the better choice is to use sphinx-external-toc.

• Change all repos to use sphinx-external-toc.
• Change all repos to use a fork of sphinx-multiversion so that recent tagged releases can use sphinx-external-toc as well.
• Add sphinxcontrib-copydirs to all repos so that examples are copied into the Sphinx source directory and that images are copied from the examples directory (and subdirectories) into the Sphinx build output.

[mikemckiernan]

Merlin

• push play-ext-toc to fork and clobber main
• confirm docs-sched-rebuild builds
• open PR

Systems

• push docs-ext-toc to fork and clobber main
• push v0.1.0-docs to fork
• confirm docs-sched-rebuild builds
• push v0.1.0-docs upstream
• open PR

Models

• push docs-ext-toc to fork and clobber main
• push v0.2.0-docs and v0.3.0-docs to fork
• confirm docs-sched-rebuild builds
• push v0.2.0docs and v0.3.0-docs upstream
• open PR

Core

• push docs-ext-toc to fork and clobber main
• confirm docs-sched-rebuild builds
• open PR

NVTabular

• push docs-ext-toc to fork and clobber main
• push v0.10.0-docs, v0.11.0-docs, v0.7.1-docs, v0.8.0-docs, v0.9.0-docs, v1.0.0-docs to fork
• confirm docs-sched-rebuild builds
• push v0.10.0-docs, v0.11.0-docs, v0.7.1-docs, v0.8.0-docs, v0.9.0-docs, v1.0.0-docs upstream
• open PR

HugeCTR

• push docs-rn to fork and clobber main
• push v3.5-docs to fork
• confirm docs-sched-rebuild builds
• push v3.5-docs upstream
• open MR

T4R

• push docs-ext-toc to fork and clobber main
• push v0.1.2-docs, v0.1.3-docs, v0.1.4-docs, v0.1.5-docs, v0.1.6-docs, v0.1.7-docs to fork
• confirm docs-sched-rebuild builds
• push v0.1.2-docs, v0.1.3-docs, v0.1.4-docs, v0.1.5-docs, v0.1.6-docs, v0.1.7-docs to upstream
• open PR

dist-embed

• push docs-ext-toc to fork and clobber main
• confirm docs-sched-rebuild builds
• open PR

template

• push docs-ext-toc to fork and clobber main
• confirm docs-sched-rebuild builds
• open PR