RTL support for guide website theme

[Raghaddahi]

Fixes #447

Description

Adds RTL support to the User Guide theme, so pages render correctly under an RTL active translation (e.g. ar). The theme was hard-wired to LTR.

Four groups of changes:

• Page direction: base.html derives dir from the active translation via get_current_language_bidi
• Mixed Arabic/English content: dir="auto" on page titles, headings, navigation, search results, footer blocks, alerts, prev/next labels and bylines, so the browser picks the direction per string. Becomes visible once real Arabic translations land (current .po files are stubs).
• Flipping directional icons: prev/next arrow, TOC arrow, and section explanation/tutorial icons mirrored via [dir='rtl'] transforms using a new svg_icon--flip-rtl opt-in class — avoids separate RTL SVG assets.
• Logo and burger placement: logical inset properties so buttons anchor to the right corner; RTL-scoped grid overrides reposition the logo right at the large breakpoint without changing LTR. Mobile menu overlay switched absolute→fixed so it stops clipping when scrolled.

Areas needing careful review

• header.scss: confirm logo mirrors right in RTL and LTR is unchanged.
• tutorial.svg: its "1 2 3" digits share the path with the list bars, so scaleX(-1) mirrors the digits backward.

Testing

Each item below shows the RTL bug before this PR, and the fix after.

1. Logo cropped on mobile

Before:

After:

2. Logo alignment (desktop)

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After:

3. Mobile menu overlay

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After:

4. Results count

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5. Flipped icons

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6. Feedback message

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7. Burger and search position

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8. dir="auto" to support mixed-language content

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After:

AI usage

AI assisted with the PR description and portions of the SCSS code (model: GLM 5.2, via opencode). Reviewed and tested by me