Make x3 docs more visible

[Flamefire]

When I open the documentation spirit 2 is shown: https://www.boost.org/doc/libs/1_86_0/libs/spirit/doc/html/index.html

Only a remark-like comment at the bottom reads:

For the newest (C++14) version, please follow to Spirit.X3

I found that a bit hard to find when looking for the "normal" documentation.

[ribbon-otter]

I would like to second this. I only learned the spirit 2 was not the most recent version because I was looking for exact source code so I could read the headers.