Clarifications for migration guides

[utensil]

Fixes #525

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.96%. Comparing base (644a32a) to head (f1825be). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #527 +/- ## ======================================= Coverage 78.95% 78.96% ======================================= Files 19 19 Lines 4400 4402 +2 ======================================= + Hits 3474 3476 +2 Misses 926 926 ```

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[utensil]

Doc build failed due to

/home/docs/checkouts/readthedocs.org/user_builds/galgebra/checkouts/527/doc/installation.rst:1: ERROR: Error in "mdinclude" directive:
unknown option: "start-after".

.. mdinclude:: ../README.md
    :start-after: <!-- begin: installation -->
    :end-before: <!-- end: installation --> [docutils]
/home/docs/checkouts/readthedocs.org/user_builds/galgebra/checkouts/527/doc/migration.rst:1: ERROR: Error in "mdinclude" directive:
unknown option: "start-after".

.. mdinclude:: ../README.md
    :start-after: <!-- begin: migration -->
    :end-before: <!-- end: migration --> [docutils]

But this option seems to be still there per https://www.docutils.org/docs/ref/rst/directives.html .

reStructuredText Directives

[utensil]

Nice, now doc build passes. The cause was incompatible theme version (jumped to 3.x.x which is not meant to be compatible), so I fixed it to the earlier one.

I should have fixed all deps to a version to avoid this happen again.