[4.2] remove submodules + remove dead analytics

[anoderay]

release-4.2 still carried the three hugo-darktable-docs-theme submodules pointing at pixlsus org repo including the dead analytics.html partial (see #868).

This converts the three theme submodules (hugo-darktable-docs-theme, hugo-darktable-docs-pdf-theme, hugo-darktable-docs-epub-theme) into plain files, same approach master already uses, then removes analytics.html and its baseof.html reference in a separate commit.

Tested against hugo server 0.88.1 (the version pinned by this branch's shell.nix). To download it to your home directory:

mkdir -p ~/hugo-0.88.1
curl -fsSL -o ~/hugo-0.88.1/hugo.tar.gz "https://github.com/gohugoio/hugo/releases/download/v0.88.1/hugo_extended_0.88.1_Linux-64bit.tar.gz"
tar xzf ~/hugo-0.88.1/hugo.tar.gz -C ~/hugo-0.88.1 hugo

Then to run it, from your local dtdocs repo root:

~/hugo-0.88.1/hugo server --disableFastRender