Update free-programming-books-et.md (one LaTex book)

[hwahyeon]

What does this PR do?

Add resource(s)

For resources

Description

Estonian translation of "The Not So Short Introduction to LaTeX"

Why is this valuable (or not)?

LaTex is useful and already exists in other language md.

How do we know it's really free?

Books under the GNU license

For book lists, is it a book? For course lists, is it a course? etc.

It a book.

Checklist:

• [x] Read our contributing guidelines.
• [x] Search for duplicates.
• [x] Include author(s) and platform where appropriate.
• [x] Put lists in alphabetical order, correct spacing.
• [x] Add needed indications (PDF, access notes, under construction).
• [x] Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!

[github-actions[bot]]

Linter failed, fix the error(s):

  free-programming-books/books/free-programming-books-et.md
  51:1-51:113  warning  Incorrect number of blank lines between last section and next heading  blank-lines-1-0-2  remark-lint

[eshellman]

Thanks, and welcome to free-programming-books!