`embed-lgpl-docs` needs post-processing to make documentation valid and solve warnings

From !725 (comment 852087), so that we don't forget :)

Documenting for future reference: in the linked MR docs are now build-tested by the regular pipeline, showing quite some warnings lately (mainly related to the addition of autolinks and intra-doc-links in rustdoc). See for example the latest run on master as of writing: https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/jobs/8363310.

• Links to preprocessor defines that do not exist anywhere in scope: [GstValueArray](GST_TYPE_ARRAY) - these should probably be #GST_TYPE_ARRAY or %GST_TYPE_ARRAY in the source;
• URLs that should have been auto-links wrapped in <>, otherwise they are not clickable;
• Links to ffi functions: [Rendering settings](ges_pipeline_set_render_settings) - this is valid if the link was ffi::ges_pipeline_set_render_settings, but the documentation should obviously link to the safe function: Self::set_render_settings.
  EDIT: This pattern seems more prevalent in the source, we probably just need to map [](<something linkable>) with C symbols just like we do for [#%] already;
• [something] pattern is detected as intra-doc-link, ie. [0,1]. These should simply be escaped as `[0,1]` in the documentation IMO.

Future improvements, not necessarily related to these warnings:

• (gir) Stop emitting Feature: `XXX` when https://github.com/gtk-rs/gir/issues/1079 is solved: feature requirements are already displayed in a nice, uniform, obvious, consistent way. No need to repeat it in text Will be fixed by https://github.com/gtk-rs/gir/pull/1086;
• C source does not use @/# everywhere, leading to missing links.

Part of these issues are probably better moved to the respective (GStreamer-XXX C repos, gir), but at least we can back-reference here and cross them off.