Extract hardcoded homepage strings to locale files (Fixes #3802)

[ablzh]

This PR resolves #3802 by extracting the remaining hardcoded English strings on the homepage into the locale system, enabling full localization for non-English users.

Specifically, the following strings were extracted to _data/locales/home/en.yml:

1. The "A Programmer's Best Friend" tagline in the Hero section and SEO meta tags (hero.tagline).
2. The "TRY!" button text (try_ruby.button_text).
3. The "Try Ruby" link text at the bottom of the section (try_ruby.bottom_link_text).

Implementation Details & Fallback Mechanism
To avoid breaking the homepage for languages that do not yet have these translations, I implemented a strict string-level English fallback using the Liquid default filter:

   1 {% assign en_home = site.data.locales.home.en %}
   2 {{ home.hero.tagline | default: en_home.hero.tagline }}

Instructions for Locale Maintainers
Other language maintainers do not need to do anything immediately; their pages will continue to display the English strings safely.

To localize these strings for your language, simply add the following keys to your respective _data/locales/home/*.yml file:

   1 hero:
   2   # ...
   3   tagline: "Your translation here"
   4
   5 try_ruby:
   6   # ...
   7   button_text: "Your translation here"
   8   bottom_link_text: "Your translation here"