docs: refine README install flow and devtool section (#656)

* docs: update README for llgo-main pipeline and debug tools

* docs: update install guide for llgo main pipeline

* docs: remove cjson package install from README

* docs: restore opening wording and mention llgo pipeline

* docs: split devtool installs under how-to-install

* docs: devtool -> Development tools

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: luoliwoshang

README.md

@@ -21,17 +21,21 @@ https://github.com/goplus/llgo?tab=readme-ov-file#how-to-install
 
 ```bash
 llgo install ./cmd/llcppg
+```
+
+For normal package generation, only the `llcppg` binary is required.
+
+### Development tools
 
-# Optional debug / troubleshooting tools:
+For debugging and isolated troubleshooting, you can optionally install these standalone tools:
+
+```bash
 llgo install ./_xtool/llcppsymg
 llgo install ./_xtool/llcppsigfetch
 llgo install ./cmd/gogensig
 go install ./cmd/llcppcfg
 ```
 
-For normal package generation, only the `llcppg` binary is required.
-The standalone tools above are kept mainly for debugging and isolated troubleshooting.
-
 
 ## Usage