Address the review in #9: improper log scales, a new-scheme tutorial, docs

[bvdmitri]

Closes #9. Each point of the review, and what this changes:

• The registry: what is it for? A new section on Inspecting rules, "What the registry is for", with an example: a module defines a node and two rules, and registered_rules, rule_coverage and a RuleNotFoundError's near misses all read its registry, while resolution, Julia's dispatch, never does. The Registry docstring points to it.

1. Purity. Pure means that a call has no effect anyone outside it can observe, apart from its result, not that it writes nothing. The docstring of ispure and Algorithms and dependencies now list what a pure rule may do (allocate, write the output buffer it is handed, reuse its scratch, draw from ctx.rng, warn or log) and what makes one impure (mutating an input or its algorithm, writing globals or files, carrying its own generator).
2. The tutorial's opening reads "builds a factor node step by step, starting from an empty module" (not "from scratch", since scratch is a term of this package).
3. A Makefile: make test as CI runs it (TEST_ALL=true, FastCholesky throwing on asymmetry), with test_args="tag:… name:… <file>"; test-fast, docs, docs-serve (LiveServer is now in docs/Project.toml), format, check-format, clean.
4. The overview shows the README's short example and points to the tutorial for the explanation.
5. "Extending the default scheme" no longer names ContinuousTransition; it explains the need through the page's own Transition node.
6. Resolution has a glossary entry (finding the rule that runs: Julia's dispatch over the methods the macros define), linked from its first use on each page; the registry sentences say that resolution does not use the registry.
7. Expectation propagation and new schemes. The glossary entry points to A node with its own algorithm, whose rule towards the input is an EP rule. A new tutorial, A new message passing scheme, writes natural-gradient message passing for y ~ Poisson(exp(x)) as an algorithm, a dependency declaration (the rule reads its own belief q[:in]) and a closed-form rule, iterated to its fixed point, citing Information Geometry of Message Passing. It needs no package beyond the docs' usual ones.
8. Log scales of improper messages. A new declaration, logscale = improper, for a message with no normalising constant, as an exact message may be: its log scale is an UndefinedLogScale with cause :improper, and require_logscale says that none exists, where an omitted logscale says that it is not known. The Log scales page has a section on improper messages, and the glossary qualifies the definition.

improper is a new public name, so this is a minor release, 1.1.0, once approved. The suite passes locally with CI's variables, and the docs build.

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