fix(import): RM-17170 Strip '-txt' endings from stub files generated

[xavierandueza]

🚥 Resolves RM-17170

🧰 Changes

We weren't handling pages that ended in .txt like mysite.com/docs/hello.txt
That ended up being:

hello-txt instead of hello for the file name and ultimately the slug.

The importer now determines these cases by looking for the .txt alongside .md, .html and more.

🧬 QA & Testing

For https://docs.warp.dev/llms.txt

BEFORE

https://docs.warp.dev/_llms-txt/getting-started.txt             → getting-started-txt
https://docs.warp.dev/_llms-txt/guides.txt                      → guides-txt
https://docs.warp.dev/_llms-txt/terminal.txt                    → terminal-txt
https://docs.warp.dev/_llms-txt/code.txt                        → code-txt
https://docs.warp.dev/_llms-txt/agent-platform.txt              → agent-platform-txt
https://docs.warp.dev/_llms-txt/knowledge-and-collaboration.txt → knowledge-and-collaboration-txt
https://docs.warp.dev/_llms-txt/enterprise.txt                  → enterprise-txt
https://docs.warp.dev/_llms-txt/reference.txt                   → reference-txt
https://docs.warp.dev/_llms-txt/support.txt                     → support-txt
https://docs.warp.dev/_llms-txt/changelog.txt                   → changelog-txt

AFTER

https://docs.warp.dev/_llms-txt/getting-started.txt             → getting-started
https://docs.warp.dev/_llms-txt/guides.txt                      → guides
https://docs.warp.dev/_llms-txt/terminal.txt                    → terminal
https://docs.warp.dev/_llms-txt/code.txt                        → code
https://docs.warp.dev/_llms-txt/agent-platform.txt              → agent-platform
https://docs.warp.dev/_llms-txt/knowledge-and-collaboration.txt → knowledge-and-collaboration
https://docs.warp.dev/_llms-txt/enterprise.txt                  → enterprise
https://docs.warp.dev/_llms-txt/reference.txt                   → reference
https://docs.warp.dev/_llms-txt/support.txt                     → support
https://docs.warp.dev/_llms-txt/changelog.txt                   → changelog