doc: Add link to ccache docs for more description

[spicyzboss]

Add description link to base_dir option in ccache.

Before

After

[DrahtBot]

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Code Coverage

For detailed information about the code coverage, see the test coverage report.

Reviews

See the guideline for information on the review process.

Type	Reviewers
Concept ACK	tdb3

If your review is incorrectly listed, please react with 👎 to this comment and the bot will ignore it on the next update.

[tdb3]

Concept ACK.
Seems beneficial to add clarification.

nit: In case the link to the ccache doc goes stale, perhaps also include the rationale from it directly in productivity.md (although I can also appreciate deferring to ccache docs for specifics because it's not Bitcoin's task to maintain other project documentation).

You _must not_ set base_dir to "/", or anywhere that contains system headers (according to the [ccache docs](https://ccache.dev/manual/latest.html#_configuration_options)), since doing this will make ccache also rewrite paths to system header files, which typically is counterproductive.

[tdb3]

Rather than change the same line in two commits, it would be better to squash those commits. (See https://github.com/tdb3/toolbox/blob/main/git/commitSquashExample.md for an example). This way the commit log is cleaner.

[maflcko]

Not sure. Is it really that hard to use a search engine? https://duckduckgo.com/?q=base_dir+ccache+docs

[spicyzboss]

I guess the explanation with link seems to make docs more complete