Link the blocking IO footnote to the IO chapter

[zjncs]

Problem

The [^blocking] footnote in src/intro.md ends with two placeholder links that were never filled in:

We'll explain what blocking IO is in chapter TODO and why you shouldn't do blocking IO in an async function in chapter TODO.

Both render as <a href="">chapter TODO</a> — dead links to the page itself — in the published book (2 empty href="" anchors in the rendered intro.html).

Change

Point the footnote at the existing guide chapter that covers exactly this material, IO and issues with blocking (src/part-guide/io.md), and merge the two sentence halves into one link since they target the same chapter.

Verification

• Base SHA: 43891ce (tip of master).
• Reproduced first: grep -c 'href=""' book/html/intro.html → 2 before the change, 0 after; the footnote now renders as <a href="part-guide/io.html">the IO chapter</a>.
• mdbook build on a combined validation branch (this change cherry-picked together with Fix async project goal link #270) completes with exit 0, including the linkcheck2 renderer.
• Note: mdbook build on this branch alone fails linkcheck2 on src/intro.md:55 — that is the pre-existing broken rust-project-goals link on master (the repo was renamed), which is fixed separately in Fix async project goal link #270. It is unrelated to and not caused by this change.
• The examples test suite is unaffected by a prose-only change (13 passed / 0 failed on the same base in Fix async project goal link #270).

Risk

Minimal: one-line documentation fix.

AI assistance disclosure

This patch was prepared by an autonomous AI coding agent at the direction of the account owner.

[zjncs]

The build and test check failure is pre-existing on master and unrelated to this change.

Evidence: the linkcheck failure reports exactly one broken URL:

error: Server returned 404 Not Found for https://github.com/rust-lang/rust-project-goals/issues/105

That is the project-goal link in src/intro.md, which still points at the old rust-lang/rust-project-goals repository on master — this PR's diff does not touch it (the diff only fills in the two [^blocking] footnote links on one line). Any branch cut from master before that link is fixed will fail this check the same way.

#270 fixes exactly that link. Once it lands, I'm happy to rebase this PR so CI runs green — or I can rebase it on top of #270's branch now if the maintainers prefer.

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