Adds an upgrading page for a host from 0.4

[johnnyt]

What

A new how-to, docs/upgrading.md, "Upgrading a host from 0.4 to 0.5", in the family's host-page shape (one ## per step, NONE where a host changes nothing, what the release added left in the CHANGELOG):

• 0.4 to 0.5: Must change NONE. May start: validate a document against the shipped draft-07 schema of the version-1 document through StatifierDatamodel.Schema.path/0 and json/0 (advisory; Index.index/1 answers as before). Worth knowing: the rule 0.5.0's StatifierDatamodel.Types documentation states for a relation wrapped around satisfies/3.
• 0.5.0 to 0.5.1: NONE; a documentation release.

mix.exs: the page is an extra in a new Upgrading group (after Explanation, the order the docs comment fixes), and it joins the package files because the README links it by a relative path (the Doc links stage refuses a README relative link to a file outside the package files). README.md: one line in the documentation map's "Do" list.

No CHANGELOG entry and no fragment: changelog.d/README.md leaves documentation out. No version bump.

Direction check

Every claim on the page was checked against the CHANGELOG and the code at the tags. git diff v0.4.0 v0.5.0 -- lib priv touches two lib files and adds the schema file: lib/statifier_datamodel/schema.ex (new, path/0 and json/0, the moduledoc's "Advisory, never admission" section), lib/statifier_datamodel/types.ex (moduledoc only: the section "A consumer's own relation above this one"), and priv/schemas/datamodel-document.schema.json (new, $schema draft-07, title "Datamodel document, version 1"); lib/statifier_datamodel/index.ex is untouched, which is what "answers exactly as before" rests on (CHANGELOG ## [0.5.0] 2026-09-26). git diff v0.5.0 v0.5.1 -- lib priv is empty (CHANGELOG ## [0.5.1] 2026-10-05: "The code is unchanged from 0.5.0"). The elixir: "~> 1.18" requirement is the same at v0.4.0 and v0.5.1. The sibling requirement claim was read from each tag's mix.exs: every statifier_blocks tag from v0.35.0 through v0.42.1, and statifier_ui v0.10.1 and v0.10.2, require {:statifier_datamodel, "~> 0.4"}. The upgrade host in statifier_examples crossed this step in PR 190 (0.4.0 to 0.5.1, suite green at each commit) and its "What a host changes" section is NONE, with no item a host must change; the page agrees. The diff removes no line outside the mix.exs lines the edit replaces: the last entry of the extras list and of groups_for_extras, each of which gains a comma, and the package files comment and list.

Test plan

• Full mix quality green on the committed tree: format, compile, credo, dialyzer, deps, docs (no warnings), doc links, tests with coverage.
• Built docs show upgrading.html under an Upgrading sidebar group, and the README's link resolves to it.
• mix hex.build --unpack puts docs/upgrading.md in the package.