Update docs

[gomoripeti]

• include xprof_core docs (use OTP 24 edoc + latest ExDoc)
• publish to hexdocs

---

Note

Modernizes documentation generation and publishing across the project.

• Replace mix-based ex_doc with rebar3_ex_doc; add gen_ex_doc and publish_docs targets using per-app rebar3 ex_doc
• New test_docs GitHub Actions job (OTP 26/Elixir 1.15) to validate docs build; remove inline docs build from test matrix
• Configure docs in rebar.config (project plugin, ex_doc options, docs profile); update hex plugin tag; drop sync from shell apps
• Add {doc, "doc"} to xprof.app.src and extensive @private/doc comment improvements and minor spec clarifications across xprof_core
• Fix docs content and examples in doc/src/querysyntax.md

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[cursor]

Hardcoded rebar3 path inconsistent with Makefile pattern

Low Severity

The new publish_docs target uses hardcoded ./rebar3 instead of $(REBAR3). Every other target in the Makefile uses $(REBAR3), which resolves to the system rebar3 or falls back to ./rebar3. This inconsistency means publish_docs will fail for developers who have rebar3 in their PATH but not as a local ./rebar3 file, unlike all other targets including the related publish target which uses $(REBAR3).