feat(template-engine): add NRG

[andriishin]

Adds NRG (Nanolaba Readme Generator) to the Template Engine section, alphabetized between Jtwig and Pebble.

Why it qualifies

Per CONTRIBUTING.md criteria:

• (c) Absolutely unique in its approach and function — every existing entry in Template Engine (Freemarker, Handlebars.java, Jamal, jstachio, jte, Jtwig, Pebble, Rocker, StringTemplate, Thymeleaf) is a general-purpose engine targeting HTML, source code or arbitrary text. NRG is the only entry purpose-built for Markdown documentation, with first-class support for generating multiple language variants (e.g. README.md and README.ru.md) from a single .src.md source — neither use case is covered by the existing entries.
• (d) Niche product that fills a gap — there is currently no Java tool in this list that targets multi-language README/documentation generation.

Distinguishing features (per PR rule 1)

• Multi-language output from a single template via ${en:'…', ru:'…'} substitution.
• File imports, custom widgets, and a built-in table-of-contents widget.
• Distributed as CLI, Maven plugin (com.nanolaba:nrg-maven-plugin), and Java library — covers both ad-hoc and build-pipeline use cases.

Compliance

• License: MIT (OSI-approved, non-viral) ✓ (criterion ii)
• English documentation ✓ (criterion iv)
• Repo: https://github.com/nanolaba/readme-generator
• Maven Central: https://central.sonatype.com/artifact/com.nanolaba/readme-generator
• Format: [NAME](LINK) - DESCRIPTION. ✓
• Alphabetical order ✓
• Single addition, single PR ✓