Move shell autocompletion documentation out to reference docs.

docs/reference/command-line/autocompletion/bash.md

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+---
+title: bash
+eleventyNavigation:
+  key: bash
+  parent: ➡️  Autocompletion
+---
+borgmatic includes an autocompletion script for
+[Bash](https://www.gnu.org/software/bash/) to support tab-completing
+command-line actions and flags. Depending on how you installed borgmatic, this
+may be enabled by default.
+
+If completions aren't enabled, start by installing the `bash-completion` Linux
+package or the
+[`bash-completion@2`](https://formulae.brew.sh/formula/bash-completion@2) macOS
+Homebrew formula. Then, install the shell completion script globally:
+
+```bash
+sudo su -c "borgmatic --bash-completion > $(pkg-config --variable=completionsdir bash-completion)/borgmatic"
+```
+
+If you don't have `pkg-config` installed, you can try the following path
+instead:
+
+```bash
+sudo su -c "borgmatic --bash-completion > /usr/share/bash-completion/completions/borgmatic"
+```
+
+Or, if you'd like to install the script for only the current user:
+
+```bash
+mkdir --parents ~/.local/share/bash-completion/completions
+borgmatic --bash-completion > ~/.local/share/bash-completion/completions/borgmatic
+```
+
+Finally, restart your shell (`exit` and open a new shell) so the completions
+take effect.

docs/reference/command-line/autocompletion/fish.md

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+---
+title: fish
+eleventyNavigation:
+  key: fish
+  parent: ➡️  Autocompletion
+---
+borgmatic includes an autocompletion script for [fish
+shell](https://fishshell.com/) to support tab-completing command-line actions
+and flags. Depending on how you installed borgmatic, this may be enabled by
+default.
+
+If completions aren't enabled, install the completions file globally:
+
+```shell
+borgmatic --fish-completion | sudo tee /usr/share/fish/vendor_completions.d/borgmatic.fish
+source /usr/share/fish/vendor_completions.d/borgmatic.fish
+```

docs/reference/command-line/autocompletion/index.md

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+---
+title: ➡️  Autocompletion
+eleventyNavigation:
+  key: ➡️  Autocompletion
+  parent: 💻 Command-line
+---
+
+borgmatic includes autocompletion scripts for various command-line shells to
+support tab-completing command-line actions and flags. Depending on how you
+installed borgmatic, this may be enabled by default. Here are the supported
+shells and how to configure borgmatic autocompletion for each:
+
+{{ collections.all | eleventyNavigation: "➡️  Autocompletion" | eleventyNavigationToHtml | replace: 'href="/reference/', 'href="/borgmatic/reference/' }}