docs: add pacman installation option for Arch Linux alongside AUR

[G36maid]

Since opencode is now in Arch Linux [extra] repository, users can install via pacman for stable releases. Updated all 35 documentation files to show both options:

• pacman -S opencode (Stable from official repo, v1.1.53)
• paru -S opencode-bin (Latest from AUR, v1.1.60)

This gives users flexibility to choose between stable tested builds or bleeding edge releases based on their needs.

What does this PR do?

update installation option for Arch Linux alongside AUR

How did you verify your code works?

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on the search results, I found a related PR:

Potential Duplicate:

• PR docs: update Arch Linux installation to use pacman #12705: "docs: update Arch Linux installation to use pacman"
  • docs: update Arch Linux installation to use pacman #12705

Why it might be related: This PR appears to be addressing the same topic - updating Arch Linux installation documentation to use pacman instead of just AUR. Your current PR (13293) seems to be building on or updating the same changes to document both pacman and AUR options across documentation files.

You may want to check if PR #12705 is already merged or closed, and whether your PR #13293 supersedes it or is a continuation of those changes.

[G36maid]

The following comment was made by an LLM, it may be inaccurate:

Based on the search results, I found a related PR:

Potential Duplicate:

* **PR [docs: update Arch Linux installation to use pacman #12705](https://github.com/anomalyco/opencode/pull/12705)**: "docs: update Arch Linux installation to use pacman"
  
  * [docs: update Arch Linux installation to use pacman #12705](https://github.com/anomalyco/opencode/pull/12705)

Why it might be related: This PR appears to be addressing the same topic - updating Arch Linux installation documentation to use pacman instead of just AUR. Your current PR (13293) seems to be building on or updating the same changes to document both pacman and AUR options across documentation files.

You may want to check if PR #12705 is already merged or closed, and whether your PR #13293 supersedes it or is a continuation of those changes.

Thanks for the flag.

Unlike replaces the AUR instruction with pacman, this PR documents both methods.

Since the official Arch extra repository may lag behind the latest release, it is better to provide users with both choices:

pacman: For official/stable releases.

AUR: For the absolute latest updates.