generator: Always update docs.md for non-sys crates

Docs were getting out of date because there's no single canonical command to regenerate them all. Since the generator is not supposed to run too often (and it can be scoped to a specific module when working by passing path(s)) it shouldn't be too bad to slow it down even further and unconditionally update docs.md files.

This MR also adds the missing GL egl/wayland/x11 documentation.

Next steps

• [gir] Improve handling of unused -o directory when building docs, but gir demanding it anyway;
• [gir] Don't take --doc-target-path as relative? Do we have a better option, @GuillaumeGomez?;
• Update gir to master and regenerate again, with @ turned into intra-doc links to Self?

Fixes #322 (closed)