Broken links in data-sources.md

[jsjtxietian]

When surfing around dev docs I found these links are broken in data-sources.md:

• ProfileEntry.h
• Profile Data Format
• Timeline C++ Implementation
• Timeline Devtools JS Server

┆Issue is synchronized with this Jira Task

[canova]

Thanks for finding these broken links! dxr was replaced by https://searchfox.org but it looks like the links are not redirected correctly.
And the timeline (docshell markers) implementation has been removed from Firefox completely. I think it makes sense to remove it from the doc. I'll update it.