update docs folder links

[Arhell]

Q	A
Is bugfix?	✔️/❌
New feature?	✔️/❌
Breaks BC?	✔️/❌

resolve https://github.com/yiisoft/yii-bootstrap5/issues/139

[what-the-diff[bot]]

PR Summary

This Pull Request contains multiple changes across different language versions of the documentation. Essentially, it updates insecure HTTP links to the secure HTTPS versions in documentation files. This contributes favourably to the overall security posture and user experience, as HTTPS ensures encrypted communication, thus offering more privacy and data security to users when they follow these links.

Specifically, the following areas have been updated:

• Bootstrap Framework Links The link to Bootstrap 4 Framework in README.md across multiple localized documentation directories has been updated for secure communication.

• Reference Links in assets-setup.md The links to Bower and NPM have been updated in assets-setup.md located in the German and Japanese guides.

• Bootstrap Documentation Links Links to Bootstrap documentation website in basic-usage.md file in multiple guides have been updated.

• Composer Links in installation.md The Composer download link has been updated to the secure version in installation.md across multiple guides.

• Bootstrap Customization Links in topics-sass.md The link to Bootstrap customization website has been updated in topics-sass.md documenting pages in several language directories.

Overall, these changes enhance the browsing security of users, leading to an improved experience for the end-users and contributors.