Add a shell completions installation guide to the docs

[owenthcarey]

Summary

weaveffi completions <shell> generates completion scripts for bash, zsh, fish, powershell, and elvish, and the README lists the command in the CLI reference table, but the docs site (docs/) never explains how to actually install the output. Users have to know each shell's convention themselves (where to put the script, what to add to .zshrc, and so on). A search for "completions" under docs/src/ comes up empty.

Proposed change

Add a short page (e.g. docs/src/reference/completions.md) with a per-shell installation snippet:

• bash (sourcing from ~/.local/share/bash-completion/completions/ or bash_completion.d)
• zsh (writing to a directory on $fpath as _weaveffi, plus the compinit note)
• fish (~/.config/fish/completions/weaveffi.fish)
• PowerShell (appending to $PROFILE)
• elvish (eval in rc.elv)

Wire the page into docs/src/SUMMARY.md so mdBook picks it up, and add a one-line pointer from the install section of docs/src/getting-started.md.

Acceptance criteria

• New page with copy-pasteable install instructions for all five shells supported by weaveffi completions
• Page is linked from docs/src/SUMMARY.md and mentioned in docs/src/getting-started.md
• Instructions verified against at least one shell locally (mention which in the PR)
• Prose follows the repo's documentation style (CMOS grammar, straight quotes, no em dashes, contractions where natural)

Pointers

• Command implementation: cmd_completions in crates/weaveffi-cli/src/main.rs
• Docs sources: docs/src/, table of contents in docs/src/SUMMARY.md

Suggested commit message

docs: add a shell completions installation guide

[SatvikMishra08]

I'd like to take this issue — please assign it to me.

I'll add a shell completions installation guide under docs/src/reference/completions.md (bash, zsh, fish, PowerShell, elvish), wire it into SUMMARY.md and getting-started.md, and verify the instructions against at least one shell locally before opening a PR.

Per CONTRIBUTING, I'll wait for assignment before opening a pull request.

[owenthcarey]

Awesome! Assigned to you.