Increase font-weight for the text in the demos in the docs

[sequba]

Context

Changing the font-weight of the text in demos from 300 to 400.

Motivated by discussion: https://handsoncode.slack.com/archives/C031GBX62SE/p1730895128146949

How did you test your changes?

local docs build

Types of changes

• Breaking change (a fix or a feature because of which an existing functionality doesn't work as expected anymore)
• New feature or improvement (a non-breaking change that adds functionality)
• Bug fix (a non-breaking change that fixes an issue)
• Additional language file, or a change to an existing language file (translations)
• Change to the documentation

Checklist:

• I have reviewed the guidelines about Contributing to HyperFormula and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement.
• My change is compliant with the OpenDocument standard.
• My change is compatible with Microsoft Excel.
• My change is compatible with Google Sheets.
• I described my changes in the CHANGELOG.md file.
• My changes require a documentation update.
• My changes require a migration guide.

[github-actions]

Performance comparison of head (a4470de) vs base (34d0a37)

                                     testName |   base |   head | change
------------------------------------------------------------------------
                                      Sheet A | 517.78 | 502.04 | -3.04%
                                      Sheet B | 168.58 | 162.35 | -3.70%
                                      Sheet T | 148.45 | 145.88 | -1.73%
                                Column ranges | 513.64 | 508.35 | -1.03%
Sheet A:  change value, add/remove row/column |  14.93 |  13.78 | -7.70%
 Sheet B: change value, add/remove row/column | 128.26 | 126.66 | -1.25%
                   Column ranges - add column | 143.57 | 145.75 | +1.52%
                Column ranges - without batch | 432.67 | 427.74 | -1.14%
                        Column ranges - batch | 107.56 | 107.41 | -0.14%