docs: rust: improve main page introducing a "Code documentation" section

Clean the "Rust" main page by introducing a 'Code documentation" section
to separate it from the rest of the text above.

In addition, introduce the "Rust code documentation" term, which may be
clearer than referring to a potentially unknown tool.

Furthermore, for the HTML case, homogenize both `rustdoc` and
non-`rustdoc` cases and use the term introduced above instead.

Then, always generate the pregenerated version part, since now there
is a section that is always generated and thus makes sense to do so.

Finally, finish the new section with a link to more details about the
Rust code documentation.

The intention is that:

  - The non-HTML case mentions the code documentation too, making it
    more prominent for readers of non-HTML docs.

  - Both HTML cases read more naturally.

  - The pregenerated version is always mentioned, since it is likely
    useful for readers of non-HTML docs.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

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