Docs: Add "Horizontal centering" subheading for .m-x-auto
Docs: Add "Horizontal centering" subheading for .m-x-auto

Refs #19102
[skip sauce]
fschumann1211/bootstrap,GerHobbelt/bootstrap,vsn4ik/bootstrap,igoynawamreh/bootstrap,abvalentine/bootstrap,zalog/bootstrap,creativewebjp/bootstrap,9-chirno/chirno,twbs/bootstrap,TheFridayInstitute/bootstrap,slavanga/bootstrap,Delagen/bootstrap,inway/bootstrap,n054/bootstrap,sealocal/bootstrap,bardiharborow/bootstrap,blockstack/blockstack-bootstrap,abvalentine/bootstrap,lemuelbarango/bootstrap,seanwu99/bootstrap,coliff/bootstrap,david-schopf/bootstrap,lemuelbarango/bootstrap,tibo66/bootstrap,edanbarak/bootstrap,blockstack/blockstack-bootstrap,gijsbotje/bootstrap,SweetProcess/bootstrap,slavanga/bootstrap,supergibbs/bootstrap,TheFridayInstitute/bootstrap,hebbet/bootstrap,abvalentine/bootstrap,Hightrack/bootstrap,GerHobbelt/bootstrap,Hemphill/bootstrap-docs,wesamco/bootstrap-rtl,realcrowd/bootstrap,frostbitten/bootstrap,supergibbs/bootstrap,slavanga/bootstrap,david-schopf/bootstrap,lipis/bootstrap,Hemphill/bootstrap-docs,m5o/bootstrap,slavanga/bootstrap,tagliala/bootstrap,frostbitten/bootstrap,tibo66/bootstrap,kvlsrg/bootstrap,Scoutski/bootstrap,n054/bootstrap,johnajacob/bootstrap,SweetProcess/bootstrap,fschumann1211/bootstrap,gijsbotje/bootstrap,9-chirno/chirno,tibo66/bootstrap,joblocal/bootstrap,JumpLinkNetwork/bootstrap-backward,edanbarak/bootstrap,johnajacob/bootstrap,jipexu/bootstrap,andresgalante/bootstrap,janseliger/bootstrap,kvlsrg/bootstrap,tomlutzenberger/bootstrap,hebbet/bootstrap,9-chirno/chirno,NCARB/bootstrap,supergibbs/bootstrap,yuyokk/bootstrap,NCARB/bootstrap,lipis/bootstrap,nice-fungal/bootstrap,fschumann1211/bootstrap,igoynawamreh/bootstrap,IdanCo/bootstrap,vsn4ik/bootstrap,joblocal/bootstrap,johnajacob/bootstrap,IdanCo/bootstrap,david-schopf/bootstrap,Scoutski/bootstrap,copperdesign/bootstrap,tomlutzenberger/bootstrap,TheFridayInstitute/bootstrap,bardiharborow/bootstrap,vsn4ik/bootstrap,hebbet/bootstrap,bardiharborow/bootstrap,fschumann1211/bootstrap,SweetProcess/bootstrap,xt0rted/bootstrap,SweetProcess/bootstrap,JumpLinkNetwork/bootstrap-backward,Scoutski/bootstrap,xt0rted/bootstrap,afuno/bootstrap,yuyokk/bootstrap,blockstack/blockstack-bootstrap,Delagen/bootstrap,Lyricalz/bootstrap,Hightrack/bootstrap,vsn4ik/bootstrap,tjkohli/bootstrap,nice-fungal/bootstrap,peterblazejewicz/bootstrap,janseliger/bootstrap,Scoutski/bootstrap,afuno/bootstrap,xt0rted/bootstrap,tomlutzenberger/bootstrap,abvalentine/bootstrap,afuno/bootstrap,Delagen/bootstrap,inway/bootstrap,tagliala/bootstrap,seanwu99/bootstrap,realcrowd/bootstrap,jipexu/bootstrap,m5o/bootstrap,zalog/bootstrap,IdanCo/bootstrap,realcrowd/bootstrap,andresgalante/bootstrap,peterblazejewicz/bootstrap,creativewebjp/bootstrap,creativewebjp/bootstrap,joblocal/bootstrap,jipexu/bootstrap,igoynawamreh/bootstrap,stanwmusic/bootstrap,lemuelbarango/bootstrap,supergibbs/bootstrap,hebbet/bootstrap,Hemphill/bootstrap-docs,JumpLinkNetwork/bootstrap-backward,kvlsrg/bootstrap,lemuelbarango/bootstrap,gijsbotje/bootstrap,andresgalante/bootstrap,wesamco/bootstrap-rtl,stanwmusic/bootstrap,andresgalante/bootstrap,bootstrapbrasil/bootstrap,bootstrapbrasil/bootstrap,stanwmusic/bootstrap,Lyricalz/bootstrap,n054/bootstrap,realcrowd/bootstrap,sealocal/bootstrap,NCARB/bootstrap,9-chirno/chirno,inway/bootstrap,Lyricalz/bootstrap,davethegr8/bootstrap,stanwmusic/bootstrap,copperdesign/bootstrap,nice-fungal/bootstrap,joblocal/bootstrap,edanbarak/bootstrap,peterblazejewicz/bootstrap,gijsbotje/bootstrap,twbs/bootstrap,zalog/bootstrap,seanwu99/bootstrap,sealocal/bootstrap,yuyokk/bootstrap,sealocal/bootstrap,NCARB/bootstrap,JumpLinkNetwork/bootstrap-backward,TheFridayInstitute/bootstrap,Hightrack/bootstrap,lipis/bootstrap,frostbitten/bootstrap,frostbitten/bootstrap,IdanCo/bootstrap,igoynawamreh/bootstrap,creativewebjp/bootstrap,edanbarak/bootstrap,davethegr8/bootstrap,lipis/bootstrap,Hemphill/bootstrap-docs,wesamco/bootstrap-rtl,seanwu99/bootstrap,n054/bootstrap,jipexu/bootstrap,afuno/bootstrap,xt0rted/bootstrap,janseliger/bootstrap,copperdesign/bootstrap,coliff/bootstrap,wesamco/bootstrap-rtl,davethegr8/bootstrap,bardiharborow/bootstrap,davethegr8/bootstrap,bootstrapbrasil/bootstrap,Delagen/bootstrap,johnajacob/bootstrap,copperdesign/bootstrap,tagliala/bootstrap,GerHobbelt/bootstrap,janseliger/bootstrap,tibo66/bootstrap,Lyricalz/bootstrap,yuyokk/bootstrap,tjkohli/bootstrap