Fix slow server startup of very big content trees

Fix slow server startup of very big content trees

As in content trees with 10 thousand of directories and more.

A benchmark with the bottle neck code in `helpers.ExtractAndGroupRootPaths`:

```
                            │ cmp20251125.bench │ fix-extractandgrouproot-14211.bench │
                            │      sec/op       │    sec/op     vs base               │
ExtractAndGroupRootPaths-10     1282818.8µ ± 8%   493.8µ ± 38%  -99.96% (p=0.002 n=6)

                            │ cmp20251125.bench │ fix-extractandgrouproot-14211.bench │
                            │       B/op        │     B/op      vs base               │
ExtractAndGroupRootPaths-10       3343.8Ki ± 0%   146.3Ki ± 0%  -95.63% (p=0.002 n=6)

                            │ cmp20251125.bench │ fix-extractandgrouproot-14211.bench │
                            │     allocs/op     │  allocs/op    vs base               │
ExtractAndGroupRootPaths-10        20.043k ± 0%    2.979k ± 0%  -85.14% (p=0.002 n=6)
```

For test project that started this (a 60k directory conent tree), the server startup with no rendering, wen from 1.5 minutes to less than 4 seconds:

```
hugop server --renderSegments none                        main ✚ ✖ ✱ ◼
Watching for changes in /Users/bep/dev/sites/hugotestsites/60k/content/{section0,section1,section10,section100,section101,section102,section103,section104,section105,section106,...}
Watching for changes in /Users/bep/dev/sites/hugotestsites/60k/layouts/_default
Watching for config changes in /Users/bep/dev/sites/hugotestsites/60k/config.toml
Start building sites …
hugo v0.153.0-DEV-7e27c303904ed8b221d6a5a4fc9a764bb7b2935b darwin/arm64 BuildDate=2025-11-25T15:02:58Z

                  │ EN
──────────────────┼────
 Pages            │  0
 Paginator pages  │  0
 Non-page files   │  0
 Static files     │  0
 Processed images │  0
 Aliases          │  0
 Cleaned          │  0

Built in 3884 ms
Environment: "development"
Serving pages from disk
Running in Fast Render Mode. For full rebuilds on change: hugo server --disableFastRender
Web Server is available at //localhost:1313/ (bind address 127.0.0.1)
Press Ctrl+C to stop
```

Note that the output may be a little different and a little more verbose than before., but the information is correct and this implementation is significantly faster and simpler.

Fixes #14211