feat(cell-merging): add hGrid and tGrid custom cell merge strategy demos

[IMinchev64]

Closes #3779

Checklist:

• check topic's TOC/menu and paragraph headings
• Include TOC topic labels in topic content has a valuable update, it's new or considered as preview\ beta
• link to other topics using ../relative/path.md
• at the References section at the end of the topic add links to topics, samples, etc
• reference API documentation instead of adding a section with API

• use valid component names - [Data] Grid, IgxSelectComponent, <igx-combo>
• use spell checker tool (VS Code, Grammarly, Microsoft Editor)
• add inline code blocks for the names of classes / tags / properties
• add language descriptor for the code blocks
• check broken links (use browser add-on)
• check if sample is working and fully visible in the topic
• check if sample is working and fully visible in the StackBlitz
• check if code blocks match the code in StackBlitz demo

• follow SEO guidelines to fill topic's metadata (SEO guidelines)

• do not resolve requested changes (leave that to the reviewer)
• add pending-localization label when the review of the PR is done
• add a member from the localization team to translate it

[ChronosSF]

@IMinchev64 , you / your team should address some stuff:

• The Hgrid samples are too big vertically with a lot of empty grid space - Either use a bigger data source that can cover the whole span or shorten the grid
• on the tree grid custom sample, try to scroll down, you'll see that it doesn't scroll the content properly. It is probably a component bug, please log it for a fix. Also the sample area is quite large and can be lowered vertically. It doesn't have to span a whole screen.

[ChronosSF]

I am releasing the docs without this and we can add it once these points are improved.

[dkamburov]

@ChronosSF this is not a product bug, we need to make a higher iframe. Opened the sample in a new window and it scrolled to the bottom:

[IMinchev64]

@ChronosSF @dkamburov I made this PR to address the sizing issues in the tree grid and hierarchical grid samples