update: CONTRIBUTING guide to document process for adding new icons

[UnschooledGamer]

Added instructions for adding new icons to the existing font family, including steps for using Icomoon and updating the style.css file.

[greptile-apps]

Greptile Summary

This PR adds a new "Adding New Icons" section to CONTRIBUTING.md that documents the end-to-end process for extending Acode's icon font family using Icomoon, and updating the corresponding style.css file. The guide fits naturally into the existing contribution documentation and covers tooling options, step-by-step Icomoon usage, and CSS class naming conventions.

Most of the issues flagged in earlier review rounds (duplicate step numbers, malformed table rows, mixed pseudo-element notation, raw HTML) appear to have been addressed. Two remaining style-level issues were found:

• The section heading ### Adding the Unicode characters to style.css. has a trailing period inconsistent with every other heading in the file.
• Step 4 of the style.css section compresses saving the CSS, extracting the zip, renaming the TTF, and copying it to the repo into a single step, which makes the guide harder to follow for first-time contributors and harder to debug if something goes wrong midway.

Confidence Score: 4/5

• This PR is safe to merge; it only modifies documentation and introduces no code changes.
• The change is documentation-only and carries no risk to the application. Previously flagged issues have been largely resolved, and the two remaining items (trailing heading period and an overly compound step) are minor style concerns that do not block contributors from following the guide correctly.
• No files require special attention beyond the two minor style nits in CONTRIBUTING.md.

Important Files Changed

Filename	Overview
CONTRIBUTING.md	Adds a new "Adding New Icons" section documenting the Icomoon-based workflow for extending the icon font. Previous issues (step numbering, table formatting, pseudo-element notation, raw HTML) appear to have been resolved; two minor style issues remain: a trailing period in a section heading and an overly dense step that combines four distinct operations.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A([Start]) --> B[Download icons.ttf\nfrom src/res/icons]
    B --> C[Open Icomoon\nStart new project]
    C --> D[Import icons.ttf]
    D --> E[Import new SVG icon]
    E --> F[Enable Show Characters\nto view Unicode]
    F --> G[Copy Unicode character]
    G --> H{More icons\nto add?}
    H -- Yes --> E
    H -- No --> I[Export → set font-family\nto code-editor-icon]
    I --> J[Download zip]
    J --> K[Open style.css\nadd .icon.icon-name:before rule]
    K --> L[Save style.css]
    L --> M[Extract zip\nrename code-editor-icon.ttf → icons.ttf]
    M --> N[Copy icons.ttf\nto src/res/icons]
    N --> O[Commit on a new branch]
    O --> Z([Done])

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_{Last reviewed commit: a49b687}

[UnschooledGamer]

Hmm, I overdid that step. Now everything seems fine to me.