Patch Antora directories only

[alandefreitas]

Changes the release script to only patch libs with directories from the Antora documentation.

This means libs won't be patched with files from the Antora build dir, such as 404.html, robots.txt, and sitemap.xml, as these are the responsibility of the boost website. Antora also used to generate and need a search-index.js file but that's been already removed in boostorg/website-v2-docs@fc1bd39.

The only problem left is _ in libs. We're discussing a solution to that.

Related to #57.

[alandefreitas]

We're discussing a solution to that.

Considering the antora directory layout, the solutions proposed so far were for the _ directory:

• Keep patching Antora into libs and leave _ in libs as part of Antora support. Cons: _ could be considered a library by some script.
• Find another directory for Antora documentation (where?) and copy all Antora docs to this new directory. Libraries using antora would update their index.html files to redirect there. Pros: when moving to Antora, libraries could temporarily access both documentation folders on boost.org before removing the old scripts. Cons: Links to documentation would change considerably.
• Move _ somewhere else (where?) and include a workaround to patch all files in Antora HTML files replacing ../_ with a relative reference to this new directory. Cons: huge unstable workaround.

[sdarwin]

There are a list of proposals here. Let me know if I can merge the existing update.

[alandefreitas]

There are a list of proposals here. Let me know if I can merge the existing update

The update in this PR is independent of the proposals. This PR is improving what we have. The proposals are about the _ directory, which this PR doesn't fix.