Temporary dartdoc workaround: remove leading href parents from extension type sidebars.

[isoos]

• This is only a temporary measure to unblock the site until extension type sidebar has a href bug after being loaded in the browser dartdoc#3663 gets fixed.
• As the sidebars are already broken, there is little risk to do a crude string replace (before sanitization).

[isoos]

Note: this has been verified locally with package:web.

[sigurdm]

Are we sure there are no other places we generate ../ that are going to break as a result of this?

[isoos]

Are we sure there are no other places we generate ../ that are going to break as a result of this?

I think the risk is very low to that:

• We are filtering on extension type sidebars via the filename.
• In addition to that, the regular sidebar does not have the ../ segment in the hrefs in the first place (hence the bug).
• I'd expect that the dartdoc fix will remove the leading ../ we are just doing it in a not-that-nice way.