This was pair-programmed with @ax-lothas

As in the standalone html builder the navigation is flattened out
for every single html page, the code needs to create a specialized toctree
for every html page.

Previously this was done by deep-copying the complete navigation toctree
and then stripping out the parts not needed on the particular page.

With this change the code only (deep)-copies the needed parts of the toctree
avoiding unnecessary copying+throwing-away

The performance improvements seems to be smaller for smaller page counts and
get bigger the more pages are involved:

+----------------------------------+-----------+-----------+-------------------+
|         830 Source Files         |   5.3.0   |  5.3.0mod |       Ratio       |
+----------------------------------+-----------+-----------+-------------------+
|          SingleThreaded          |  49.137 s |  43.517 s | 0.885625903087287 |
| Parallel (auto, 16 logical CPUs) |  13.334 s |  12.849 s | 0.963626818659067 |
+----------------------------------+-----------+-----------+-------------------+

+----------------------------------+-----------+-----------+-------------------+
|        6166 Source Files         |   5.3.0   |  5.3.0mod |       Ratio       |
+----------------------------------+-----------+-----------+-------------------+
|          SingleThreaded          | 907.497 s | 521.945 s | 0.575147906825036 |
| Parallel (auto, 16 logical CPUs) | 195.400 s | 150.139 s | 0.768367451381781 |
+----------------------------------+-----------+-----------+-------------------+

# Conflicts:
#	sphinx/environment/adapters/toctree.py