Only create LazyContentProvider for the non-rendering Site

Only create LazyContentProvider for the non-rendering Site

Which saves a fair amound of allocations:

```
gobench --package ./hugolib --bench "SiteNew/Regular_D" --base master
```

Before:

```
name                                  old time/op    new time/op    delta
SiteNew/Regular_Deep_content_tree-10    40.7ms ± 3%    41.2ms ± 1%    ~     (p=0.343 n=4+4)

name                                  old alloc/op   new alloc/op   delta
SiteNew/Regular_Deep_content_tree-10    27.7MB ± 0%    28.8MB ± 0%  +3.76%  (p=0.029 n=4+4)

name                                  old allocs/op  new allocs/op  delta
SiteNew/Regular_Deep_content_tree-10      304k ± 0%      329k ± 0%  +8.07%  (p=0.029 n=4+4)
```

After:

```
name                                  old time/op    new time/op    delta
SiteNew/Regular_Deep_content_tree-10    34.2ms ± 1%    34.7ms ± 1%    ~     (p=0.114 n=4+4)

name                                  old alloc/op   new alloc/op   delta
SiteNew/Regular_Deep_content_tree-10    27.7MB ± 0%    28.1MB ± 0%  +1.38%  (p=0.029 n=4+4)

name                                  old allocs/op  new allocs/op  delta
SiteNew/Regular_Deep_content_tree-10      304k ± 0%      314k ± 0%  +3.03%  (p=0.029 n=4+4)
```

Updates #8919