Implement /explain on vulnerability page [GitLab]

Context

This issue refers to slash commands in GitLab Duo Chat. This issue is specific to chat use within GitLab, not the IDEs

Problem to solve

Explain vulnerability has a specific slash command /vulnerability_explain. The guidelines for Duo Chat slash commands are moving towards slash commands being solution agnostic. The long-term vision is that the user could type only /explain on a variety of pages and the chat would provide an explanation.

Scope of this issue

• Refactor existing /vulnerability_explain to /explain
• While on the vulnerability page, /explain works without arguments (e.g. user does not need to provide a URL)
• While outside of the vulnerability page, the user could get a vulnerability explained with /explain+<vulnerability URL> (TBD: should this be handled in a separate issue?)
• The user can add free text, if they choose, e.g. /explain+in 5 bullet points

Related work FYI (outside of scope of this issue)

• Slash command UI on GitLab
• Guide the user to provide all required information in slash command