Document the first-launch OS warnings in the tutorial and README

[AlexisJanin]

Closes #94.

The standalone bundles are not code-signed, so every platform interrupts the very first launch. Those workarounds lived only in the GitHub release bodies, so anyone reaching the project through the repository hit a blocked launch with no documented way past it.

Wording is lifted from the tested v1.1.0 release body rather than rewritten, as the issue asked.

docs/user_guide/tutorial.md — primary target

The issue asked for a "First launch" subsection under installation, but the tutorial has no installation section — it opens at Launching the Application already assuming an unzipped bundle. So the new ## First launch sits inside that section, immediately before Starting the App, which is where it falls chronologically. No download table or install instructions were added: this file ships as the PDF inside the bundle, so its reader has already downloaded.

• ### Windows — first-launch elevation (right-click → Run as administrator) and the SmartScreen unknown-publisher warning, with the one-time / survives-reboot note.
• ### macOS (Apple Silicon) — xattr -d com.apple.quarantine on the .zip before unzipping, the browser auto-unzip heads-up, and the no-Intel-build pointer to PyPI.
• ### Linux — unzip + chmod +x.

Two smaller edits in Starting the App:

• "double-click it" directly contradicted the Windows elevation step, so it now points at First launch for the first run.
• Added how to close the app (it runs in the terminal window that opened with it), which the issue listed but had no other home.

README.md — shorter form

The same instructions in a collapsible <details> block under the pre-built download table: three per-platform bullets with the exact commands, plus the closing-the-app line. 11 lines.

Collapsible rather than inline because README readers mostly install from PyPI or source, while bundle users are the ones who read the tutorial — this keeps the page self-sufficient for a downloaded bundle without flooding it. The content is still greppable and GitHub-searchable inside <details>.

The block is deliberately README-only: build_pdf.sh feeds tutorial.md to pandoc, and raw HTML is dropped on the LaTeX path, so a <details> there would vanish from the bundled PDF.

Verification

• The issue's own check — grep -E "SmartScreen|quarantine|Run anyway" — returned nothing on either file before; now 3 hits in tutorial.md, 2 in README.md.
• pandoc docs/user_guide/tutorial.md -t html parses with no warnings, and the new [First launch](#first-launch) cross-reference resolves (id="first-launch" is generated as expected), so the link works in the PDF.
• tests/plot_types/test_plot_type_is_documented.py scans tutorial.md headings for plot-type names; it asserts presence, so the new headings cannot affect it.

Acceptance criteria

• A user following only README.md can get a downloaded bundle running on Windows, macOS and Linux without consulting a release page.
• The same instructions are in tutorial.md, so the bundled PDF carries them from the next release onward.

Not in scope

• The committed ClinicalScope_UserGuide.pdf is untouched — it is rebuilt once per release (RELEASING.md step 1), so it will lag until then. The issue notes this is expected.
• Why Windows needs elevation exactly once is still open, per the issue's own note.

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