docs: migrate README.md / README.ja.md to a single NRG template

[andriishin]

Why

README.md and README.ja.md are hand-maintained side-by-side today and have drifted in a handful of places:

• The EN file documents two long sections — Code Quality Tools and Contributing Guidelines — that are not in the JA file.
• The "AuthleteApi Settings" sub-heading is #### in EN but ### in JA.
• The reference links [5] and [10] legitimately differ between files (JA points to /ja/developers/overview/ and to a Qiita article instead of the Medium one).

This PR migrates both files to a single source-of-truth template using Nanolaba Readme Generator (NRG) — a small Java 8 Maven plugin that turns one README.src.md into N localized README files on every build. The two existing files become regenerated artifacts: editing them by hand is no longer necessary and a comment header at the top warns about it.

What changes

• New: README.src.md — the unified source. Lines specific to one language are tagged with <!--en--> / <!--ja-->; per-language section titles use inline ${en:'…', ja:'…'}. The two diverging reference links are kept distinct via NRG property overrides (<!--@ref5.ja=…--><!--@ref10.ja=…-->).
• Modified: pom.xml — adds com.nanolaba:nrg-maven-plugin:1.1 to <build><plugins> (immediately after maven-compiler-plugin) so mvn compile regenerates both READMEs.
• Modified: README.md (+4 lines), README.ja.md (+27/-10 lines, all cosmetic) — regenerated. EN gets only a 2-line auto-generated header at the top. JA gets the same header plus a few cosmetic differences in setext underline length (------- vs --------) and a couple of extra blank lines around section breaks — both render identically on GitHub.

Verification

Locally, regenerating with NRG 1.1 from Maven Central against current master (commit 6d5409e, 4.44-SNAPSHOT) produces files that diff against the originals only by:

1. A 2-line header noting the file is auto-generated (top of each output).
2. In README.ja.md: setext heading underlines are normalised to a single shared length per heading level; this is purely cosmetic — markdown renderers treat any number of dashes as a valid H2 underline.

Total functional content change: zero. Tested with Java 8 and nrg-maven-plugin:1.1 from Maven Central.

What's intentionally NOT in this PR

• Translation alignment. The drift between languages (Code Quality Tools / Contributing Guidelines sections present only in EN, the ####/### heading-level mismatch) is preserved as-is. This PR is strictly mechanical so review is just "diff says the right things". Surfacing those sections to JA readers is a one-line addition per missing piece in README.src.md once the template is in place.
• CI hardening. <check>false</check> is set so the build does not fail on README drift today. A follow-up can set it to true once the workflow is comfortable with regenerating before commit.

How to regenerate locally

After this PR is merged:

mvn compile

(or mvn nrg:create-files for just the README step, skipping the rest of the build).

To preview without touching the plugin first:

curl -L -o nrg.jar https://repo1.maven.org/maven2/com/nanolaba/readme-generator/1.1/readme-generator-1.1.jar
java -jar nrg.jar -f README.src.md

About NRG

com.nanolaba:readme-generator — open-source, Apache 2.0, Java 8+. Ships as a CLI, Maven plugin, GitHub Action, and library. ~17 lines of XML in pom.xml, no runtime dependency on the plugin once READMEs are committed.