Resolve "Issue on page /numerical_methods/8-boundary-value-problem.html"

Closes #75

See merge request !395

Fix some typos in chapter 8

See merge request !427

fixed some typos

See merge request !424

Week 2.8 Preparation

See merge request !423

Update learn-programming

See merge request !419

This reverts commit 36f2d3e8.

This reverts commit b2825ea9.

This reverts commit f29d602a.

This reverts commit 42a73b89.

This reverts commit fa3a190f.

This reverts commit c6b2c71d.

This reverts commit 15152471.

This reverts commit a82c9ad5.

This reverts commit 5c78d815.

This reverts commit 3394770c.

This reverts commit 2ce80d0e.

include _ext directory

See merge request !418

Main

See merge request !417