docs: add high-level overview of regzbot

[alanpeixinho]

Description

I’ve added documentation to provide high level context (targeted at new collaborators) on how Regzbot operates under the hood.

Summary:

• Added a high-level overview covering the bot's core logic and email-command parsing.
• Documented the relationship between tracked Git trees (Mainline/Next/Stable).
• Included a Codebase Organization table to help contributors navigate the Python modules.
• Mapped out the pluggable report sources (GitHub, GitLab, Bugzilla, Lore).

[tales-aparecida]

LGTM no notes

[tales-aparecida]

@alanpeixinho pls add you sign off in the commit