Add mindtouch offliner for libretexts.org

[benoit74]

Rationale

We have a new scraper.

I propose to create:

• one new offliner ...
• one new category mindtouch
• two new warehouse paths libretexts (for libretexts.org ZIMs) and mindtouch (for other mindtouch ZIMs we might create)
• one new platform libretexts to limit to 1 job at a time on the whole zimfarm

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.41%. Comparing base (fa98354) to head (63a5880).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1038      +/-   ##
==========================================
+ Coverage   88.35%   88.41%   +0.06%     
==========================================
  Files          97       98       +1     
  Lines        5417     5447      +30     
==========================================
+ Hits         4786     4816      +30     
  Misses        631      631              

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[benoit74]

@kelson42 we need your feedback on categories / warehouse path at least ... not on the code itself obviously

[rgaudin]

LGTM ; no opinion on the values

[benoit74]

Warehouse paths created manually on the receiver.