Translate untranslated strings in Czech (cs) locale

[colinkiama]

The Czech locale file at .vitepress/locales/cs.js has the following keys still left in English and need translating:

• current_versions: "Current Versions"
• install_how: "How to install"
• stable: "Stable"
• stable_lts: "Stable Long-term Support"
• devel: "Development"
• roadmap: "Roadmap"

The following keys were added recently (in #249, the "Why Vala?" bullet point section) and also need translating:

• why_vala_syntax: "Clear and familiar syntax - Based on C, C# and Java"
• why_vala_oop: "Object-oriented when you need it - Built-in support for classes, interfaces, inheritance and more, but you can also just use procedural programming features instead."
• why_vala_memory: "Automatic memory management without overhead - Vala uses ref counting by default, so you won't have to worry about managing memory in most programs"
• why_vala_c_interop: "C interoperability - Use GObject-based C Libraries via binding files or call C functions directly from Vala"
• why_vala_native: "Compiles to native binaries - Extremely fast program execution times and no virtual machine or runtime environment is required to be installed for execution"
• why_vala_modern_features: "Modern language features - Type inference, null safety, generics, lambdas, async/await"
• why_vala_gobject: "First-class <a href="https://docs.gtk.org/gobject/\">GObject and <a href="https://www.gtk.org/\">GTK support - Optional, but unmatched when you need it"

You can compare against .vitepress/locales/en.js for the English source text.