Add --share-log flag to upload install.log to paste.rs

[Softer]

Replaces the broken curl ... https://0x0.st hint with an integrated archinstall --share-log command that uploads /var/log/archinstall/install.log to paste.rs and prints a shareable URL. Also updates the same hint in the README, the bug report docs, and the GitHub bug issue template, so all four user-facing references now point to the working command.

Refs #4396 (0x0.st has disabled uploads). Supersedes #4507, which removes the hint without offering a replacement.

Why paste.rs

• Text-oriented pastebin with extension-based syntax highlighting, so install.log renders as a readable page rather than a raw download
• 10 MiB upload limit (well above realistic install.log sizes)
• Curl-friendly, no account, no server-side state to maintain

The endpoint is referenced exactly once - a constant in archinstall/lib/share_log.py. If a self-hosted paste service is set up later, or 0x0.st returns, swapping it is a one-line change.

Behavior

$ archinstall --share-log
About to upload /var/log/archinstall/install.log (12345 bytes) to https://paste.rs 
The log may contain hostname, mirror URLs, package list and partition layout. The uploaded paste is public.
Continue? [y/N]: y 
https://paste.rs/abc.def 

• All prompts, warnings and errors go to stderr
• Only the final URL goes to stdout, so it can be piped (archinstall --share-log | xclip -sel clip)
• The function exits early before _log_sys_info() and uses bare print() instead of info()/warn(), so the file uploaded matches the file on disk byte-for-byte (no "About to upload..." line gets injected into the log)
• Explicit y/N confirmation; default is N
• Privacy warning lists what the log can contain (hostname, mirror URLs, package list, partition layout)
• File-not-found, empty file, file > 10 MiB, network failure and unexpected response are each handled with a clear message and non-zero exit

Test plan

• archinstall --share-log on a live ISO - URL is printed and the paste matches install.log byte-for-byte
• archinstall --share-log answering n - exits cleanly without uploading
• archinstall --share-log with install.log missing - reports the error and exits non-zero
• archinstall --share-log with no network - reports the error and exits non-zero
• Trigger an install error and confirm the new hint is shown in the failure message

[Torxed]

Just note that I started a conversation about an arch official channel for posting these logs. I created a ticket to track the progress on gitlab@archlinux #833

[Softer]

We can change --share-log behavior transparently for users any time we need without changing user's expectation. :)