docs: document DENO_INSTALL env var on installation page

[lunadogbot]

Summary

Adds a new "Customizing the install directory" sub-section to runtime/getting_started/installation.md that documents the DENO_INSTALL environment variable supported by the official deno_install shell and PowerShell scripts. Includes a short cross-reference from the intro paragraph so readers see the option before they run the installer.

This was requested by a reader of the installation page who pointed out that DENO_INSTALL is not currently mentioned anywhere on the page, even though the page links to deno_install (which honors it).

What's new on the page

• A note in the intro paragraph that mentions the default install location and links down to the new sub-section.
• A ### Customizing the install directory sub-section under "Download and install" with:
  • An explanation that the binary lands at \$DENO_INSTALL/bin.
  • A macOS/Linux example using curl -fsSL https://deno.land/install.sh | DENO_INSTALL=/opt/deno sh. The variable is placed before sh (not before curl) so that the installer process actually sees it — DENO_INSTALL=... curl ... | sh would only export the variable to curl, not to the piped shell, which is a subtle footgun worth getting right in the docs.
  • A Windows/PowerShell example using \$env:DENO_INSTALL.
  • A reminder to add the new bin directory to PATH and a pointer to the deno_install README as the canonical reference.

Behavior was verified by reading the current install.sh (defaults deno_install="\${DENO_INSTALL:-\$HOME/.deno}") and install.ps1 (\$DenoInstall = \$env:DENO_INSTALL) served from deno.land.

deno fmt was run on the file.

Closes #3047
Closes bartlomieju/orchid-inbox#55

cc @bartlomieju

Test plan

• CI passes (link checker, formatting).
• Render runtime/getting_started/installation.md locally with deno task serve and confirm the new sub-section reads cleanly and the anchor link from the intro paragraph resolves to it.

[fibibot]

Verified against the live install.sh (deno_install="${DENO_INSTALL:-$HOME/.deno}", bin_dir="$deno_install/bin") and install.ps1 ($BinDir = if ($DenoInstall) { "${DenoInstall}\bin" } else { "${Home}\.deno\bin" }) — the $DENO_INSTALL/bin layout, the macOS/Linux placement of DENO_INSTALL= before sh (not before curl), and the Windows $env:DENO_INSTALL pattern are all accurate. Anchor #customizing-the-install-directory resolves to the new H3.

Holding approval until CI is green.

• nit: last_modified: 2026-03-05 should bump to today — this adds a new sub-section, not a typo fix.

[fibibot]

Only change since prior review is the last_modified bump — nit addressed. Holding APPROVE until lint and link check is green.

[lunadogbot]

The lint and link check failure here is pre-existing and unrelated to this PR's diff. The build errors on /styleguide/typography.mdx with:

TypeError: Cannot resolve module "lume/jsx-runtime": relative URL with a cannot-be-a-base base
at MDXEngine.render (lume@3.1.2/plugins/mdx.ts:96:21)

This PR only touches runtime/getting_started/installation.md. The original CI run also failed (timed out at 6h after presumably the same error), and the same lint and link check job is currently CANCELLED on recent merged PRs (#3108, #3106) and on the most recent main pushes — so this looks like a repo-wide MDX/lume regression in the styleguide build, not anything from this change. The deploy/deno/docs build, which renders the actual site, passed cleanly.

cc @bartlomieju — happy to leave it to you whether to merge through this or wait for a styleguide fix first.

[bartlomieju]

@lunadogbot needs a merge and resolve confclits

[fibibot]

Diff against my last reviewed sha is the last_modified bump plus the unrelated "command not found" section pulled in from main via the merge — the PR's own contribution is unchanged. Lint and deploy/deno/docs both green. Approving.

[fibibot]

@bartlomieju this is ready to merge