doc: better way to search for flags

Is your feature request related to a problem? Please describe

Currently, the search function does not work on command line flags. For example, when a user want check the flag definition of -relabelConfig, he/she must go to either vmagent or VictoriaMetrics single-node page, and use the search function of the browser (ctrl+f).

This brings extra difficulties for new users who are not familiar with the document structure of VictoriaMetrics, and strictly speaking, it is not the most convenient for experienced users either.

Describe the solution you'd like

I suggest to make command line flags searchable on all pages. A command line flag must be indexed and appear in search result.

Ideally, the command line flag and description should be parsed and formatted correctly in the search result.

Describe alternatives you've considered

There might be other way to improve this, and the common goal should be: user can find everything he need about a flag on any page.

Additional information

Originally from this public question: https://victoriametrics.slack.com/archives/CGZF1H6L9/p1762358837043459