style: add background image and update css

[KJAyano]

Added palworld-bg.png as background image and updated extra.css for better design. Also updated mkdocs.yml accordingly.

[Zvendson]

Wow, thank you for the update. I really like the design added with this PR. I checked the last commit locally and found one concrete regression plus a few cleanup issues that should be addressed before merge.

Main issue

• Remove navigation.instant, or otherwise resolve its incompatibility with the current mkdocs-static-i18n language switcher setup.
  See:

WARNING - mkdocs_static_i18n: mkdocs-material language switcher contextual link is not compatible with theme.features = navigation.instant

This warning is introduced by this commit. The previous commit builds successfully with mkdocs build --strict.
navigation.instant was added in mkdocs.yml in this PR, and that is what triggers the warning with the existing i18n configuration.

Additional cleanup

• Remove or explain the CSS classes added in docs/stylesheets/extra.css that do not appear to be used by the current docs, such as .md-card, .md-btn, .md-hero, .md-breadcrumbs, .content-section, and .md-button--back-to-top. There might be a few more unused classes.
• Consolidate duplicated / overlapping CSS rules to make the stylesheet easier to maintain. There are multiple definitions for selectors such as .md-search__form, .md-content, .md-nav__link, and heading styles.
• Align the footer configuration with the styling. navigation.footer is enabled in mkdocs.yml, but the footer is hidden in extra.css, which is contradictory.

Notes

• No unnecessary package dependencies were introduced.
• The visual direction is clear, but the implementation is heavier than it needs to be in its current form.

Because this commit introduces a new strict-build failure and adds a fair amount of unused / overlapping CSS, I do not think it is ready to merge yet.

Please run mkdocs build --strict before resubmitting the changes.

[KJAyano]

Fixed, all points addressed. Verified with mkdocs build --strict builds cleanly. Ready for re-review.