doxygen: reorganize directory structure

The original directory structure was too flat and
inconvenient to view.

The directory is now reorganized according to the
first-level chapters, and some sub-chapters are
moved to the second-level directory.

Note: documentation/env is not moved to
documentation/4.tool because there are too many
external places that reference this path,
especially in bsp, so this directory path remains
as it is.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

.gitee/PULL_REQUEST_TEMPLATE.en.md

@@ -27,4 +27,4 @@ As part of this pull request, I've considered the following:
 - [ ] All modifications to BSP are justified and do not affect other components or BSPs.
 - [ ] I've commented appropriately where code is tricky.
 - [ ] Code in this PR is of high quality.
-- [ ] This PR complies with [RT-Thread code specification](https://gitee.com/rtthread/rt-thread/blob/master/documentation/contribution_guide/coding_style_en.md)
+- [ ] This PR complies with [RT-Thread code specification](https://gitee.com/rtthread/rt-thread/blob/master/documentation/7.contribution/coding_style_en.md)

.gitee/PULL_REQUEST_TEMPLATE.zh-CN.md

@@ -27,4 +27,4 @@
 - [ ] 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或
 - [ ] 对难懂代码均提供对应的注释
 - [ ] 本拉取/合并请求代码是高质量的
-- [ ] 本拉取/合并符合[RT-Thread代码规范](https://gitee.com/rtthread/rt-thread/blob/master/documentation/contribution_guide/coding_style_cn.md)
+- [ ] 本拉取/合并符合[RT-Thread代码规范](https://gitee.com/rtthread/rt-thread/blob/master/documentation/7.contribution/coding_style_cn.md)