Export target docs (#5812)

Co-authored-by: Martin Haug <3874949+reknih@users.noreply.github.com>

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+PDF files focus on accurately describing documents visually, but also have
+facilities for annotating their structure. This hybrid approach makes
+them a good fit for document exchange: They render exactly the same on every
+device, but also support extraction of a document's content and structure (at
+least to an extent). Unlike PNG files, PDFs are not bound to a specific
+resolution. Hence, you can view them at any size without incurring a loss of
+quality.
+
+# PDF standards
+The International Standards Organization (ISO) has published the base PDF
+standard and various standards that extend it to make PDFs more suitable for
+specific use-cases. By default, Typst exports PDF 1.7 files. Adobe Acrobat 8 and
+later as well as all other commonly used PDF viewers are compatible with this
+PDF version.
+
+## PDF/A
+Typst optionally supports emitting PDF/A-conformant files. PDF/A files are
+geared towards maximum compatibility with current and future PDF tooling. They
+do not rely on difficult-to-implement or proprietary features and contain
+exhaustive metadata. This makes them suitable for long-term archival.
+
+The PDF/A Standard has multiple versions (_parts_ in ISO terminology) and most
+parts have multiple profiles that indicate the file's conformance level.
+Currently, Typst supports these PDF/A output profiles:
+
+- PDF/A-2b: The basic conformance level of ISO 19005-2. This version of PDF/A is
+  based on PDF 1.7 and results in self-contained, archivable PDF files.
+
+- PDF/A-3b: The basic conformance level of ISO 19005-3. This version of PDF/A is
+  based on PDF 1.7 and results in archivable PDF files that can contain
+  arbitrary other related files as [attachments]($pdf.embed). The only
+  difference between it and PDF/A-2b is the capability to embed
+  non-PDF/A-conformant files within.
+
+When choosing between exporting PDF/A and regular PDF, keep in mind that PDF/A
+files contain additional metadata, and that some readers will prevent the user
+from modifying a PDF/A file. Some features of Typst may be disabled depending on
+the PDF standard you choose.
+
+# Exporting as PDF
+## Command Line
+PDF is Typst's default export format. Running the `compile` or `watch`
+subcommand without specifying a format will create a PDF. When exporting to PDF,
+you have the following configuration options:
+
+- Which PDF standards Typst should enforce conformance with by specifying
+  `--pdf-standard` followed by one or multiple comma-separated standards. Valid
+  standards are `1.7`, `a-2b`, and `a-3b`. By default, Typst outputs
+  PDF-1.7-compliant files.
+
+- Which pages to export by specifying `--pages` followed by a comma-separated
+  list of numbers or dash-separated number ranges. Ranges can be half-open.
+  Example: `2,3,7-9,11-`.
+
+## Web App
+Click the quick download button at the top right to export a PDF with default
+settings. For further configuration, click "File" > "Export as" > "PDF" or click
+the downwards-facing arrow next to the quick download button and select "Export
+as PDF". When exporting to PDF, you have the following configuration options:
+
+- Which PDF standards Typst should enforce conformance with. By default, Typst
+  outputs PDF-1.7-compliant files. Valid additional standards are `A-2b` and
+  `A-3b`.
+
+- Which pages to export. Valid options are "All pages", "Current page", and
+  "Custom ranges". Custom ranges are a comma-separated list of numbers or
+  dash-separated number ranges. Ranges can be half-open. Example: `2,3,7-9,11-`.
+
+# PDF-specific functionality
+Typst exposes PDF-specific functionality in the global `pdf` module. See below
+for the definitions it contains.