Get the whole page tree and process options to filter it

[benoit74]

Changes:

• build a tree of website pages, with links between them (parent and children, since the tree is exposed like this - a node can have only one parent)
• filter this tree of pages based on CLI input arguments (useful at least for debug, but I see use cases for production as well, one might be interested in only one course for instance)

Nota: to be reviewed after https://github.com/openzim/libretexts/pull/4 is merged

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 59.25926% with 33 lines in your changes missing coverage. Please review.

Project coverage is 47.54%. Comparing base (e7a408c) to head (35b5448). Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
scraper/src/libretexts2zim/client.py	57.14%	23 Missing and 1 partial :warning:
scraper/src/libretexts2zim/processor.py	72.72%	6 Missing :warning:
scraper/src/libretexts2zim/entrypoint.py	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5 +/- ## ========================================== + Coverage 44.94% 47.54% +2.60% ========================================== Files 7 7 Lines 336 408 +72 Branches 32 40 +8 ========================================== + Hits 151 194 +43 - Misses 184 212 +28 - Partials 1 2 +1 ```

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